Struct std::collections::BinaryHeap

pub struct BinaryHeap<T, A = Global>
where
    A: Allocator,
{ /* private fields */ }

A priority queue implemented with a binary heap.

This will be a max-heap.

It is a logic error for an item to be modified in such a way that the item’s ordering relative to any other item, as determined by the Ord trait, changes while it is in the heap. This is normally only possible through interior mutability, global state, I/O, or unsafe code. The behavior resulting from such a logic error is not specified, but will be encapsulated to the BinaryHeap that observed the logic error and not result in undefined behavior. This could include panics, incorrect results, aborts, memory leaks, and non-termination.

As long as no elements change their relative order while being in the heap as described above, the API of BinaryHeap guarantees that the heap invariant remains intact i.e. its methods all behave as documented. For example if a method is documented as iterating in sorted order, that’s guaranteed to work as long as elements in the heap have not changed order, even in the presence of closures getting unwinded out of, iterators getting leaked, and similar foolishness.

Examples

use std::collections::BinaryHeap;

// Type inference lets us omit an explicit type signature (which
// would be `BinaryHeap<i32>` in this example).
let mut heap = BinaryHeap::new();

// We can use peek to look at the next item in the heap. In this case,
// there's no items in there yet so we get None.
assert_eq!(heap.peek(), None);

// Let's add some scores...
heap.push(1);
heap.push(5);
heap.push(2);

// Now peek shows the most important item in the heap.
assert_eq!(heap.peek(), Some(&5));

// We can check the length of a heap.
assert_eq!(heap.len(), 3);

// We can iterate over the items in the heap, although they are returned in
// a random order.
for x in &heap {
    println!("{x}");
}

// If we instead pop these scores, they should come back in order.
assert_eq!(heap.pop(), Some(5));
assert_eq!(heap.pop(), Some(2));
assert_eq!(heap.pop(), Some(1));
assert_eq!(heap.pop(), None);

// We can clear the heap of any remaining items.
heap.clear();

// The heap should now be empty.
assert!(heap.is_empty())

A BinaryHeap with a known list of items can be initialized from an array:

use std::collections::BinaryHeap;

let heap = BinaryHeap::from([1, 5, 2]);

Min-heap

Either core::cmp::Reverse or a custom Ord implementation can be used to make BinaryHeap a min-heap. This makes heap.pop() return the smallest value instead of the greatest one.

use std::collections::BinaryHeap;
use std::cmp::Reverse;

let mut heap = BinaryHeap::new();

// Wrap values in `Reverse`
heap.push(Reverse(1));
heap.push(Reverse(5));
heap.push(Reverse(2));

// If we pop these scores now, they should come back in the reverse order.
assert_eq!(heap.pop(), Some(Reverse(1)));
assert_eq!(heap.pop(), Some(Reverse(2)));
assert_eq!(heap.pop(), Some(Reverse(5)));
assert_eq!(heap.pop(), None);

Time complexity

push	pop	peek/peek_mut
O(1)~	O(log(n))	O(1)

The value for push is an expected cost; the method documentation gives a more detailed analysis.

Implementations

impl<T> BinaryHeap<T>

where T: Ord,

pub fn new() -> BinaryHeap<T>

Creates an empty BinaryHeap as a max-heap.

Examples

Basic usage:

use std::collections::BinaryHeap;
let mut heap = BinaryHeap::new();
heap.push(4);

pub fn with_capacity(capacity: usize) -> BinaryHeap<T>

Creates an empty BinaryHeap with at least the specified capacity.

The binary heap will be able to hold at least capacity elements without reallocating. This method is allowed to allocate for more elements than capacity. If capacity is 0, the binary heap will not allocate.

Examples

Basic usage:

use std::collections::BinaryHeap;
let mut heap = BinaryHeap::with_capacity(10);
heap.push(4);

impl<T, A> BinaryHeap<T, A>

where T: Ord, A: Allocator,

pub fn new_in(alloc: A) -> BinaryHeap<T, A>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Creates an empty BinaryHeap as a max-heap, using A as allocator.

Examples

Basic usage:

#![feature(allocator_api)]

use std::alloc::System;
use std::collections::BinaryHeap;
let mut heap = BinaryHeap::new_in(System);
heap.push(4);

pub fn with_capacity_in(capacity: usize, alloc: A) -> BinaryHeap<T, A>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Creates an empty BinaryHeap with at least the specified capacity, using A as allocator.

The binary heap will be able to hold at least capacity elements without reallocating. This method is allowed to allocate for more elements than capacity. If capacity is 0, the binary heap will not allocate.

Examples

Basic usage:

#![feature(allocator_api)]

use std::alloc::System;
use std::collections::BinaryHeap;
let mut heap = BinaryHeap::with_capacity_in(10, System);
heap.push(4);

pub fn peek_mut(&mut self) -> Option<PeekMut<'_, T, A>>

Returns a mutable reference to the greatest item in the binary heap, or None if it is empty.

Note: If the PeekMut value is leaked, some heap elements might get leaked along with it, but the remaining elements will remain a valid heap.

Examples

Basic usage:

use std::collections::BinaryHeap;
let mut heap = BinaryHeap::new();
assert!(heap.peek_mut().is_none());

heap.push(1);
heap.push(5);
heap.push(2);
{
    let mut val = heap.peek_mut().unwrap();
    *val = 0;
}
assert_eq!(heap.peek(), Some(&2));

Time complexity

If the item is modified then the worst case time complexity is O(log(n)), otherwise it’s O(1).

pub fn pop(&mut self) -> Option<T>

Removes the greatest item from the binary heap and returns it, or None if it is empty.

Examples

Basic usage:

use std::collections::BinaryHeap;
let mut heap = BinaryHeap::from([1, 3]);

assert_eq!(heap.pop(), Some(3));
assert_eq!(heap.pop(), Some(1));
assert_eq!(heap.pop(), None);

Time complexity

The worst case cost of pop on a heap containing n elements is O(log(n)).

pub fn push(&mut self, item: T)

Pushes an item onto the binary heap.

Examples

Basic usage:

use std::collections::BinaryHeap;
let mut heap = BinaryHeap::new();
heap.push(3);
heap.push(5);
heap.push(1);

assert_eq!(heap.len(), 3);
assert_eq!(heap.peek(), Some(&5));

Time complexity

The expected cost of push, averaged over every possible ordering of the elements being pushed, and over a sufficiently large number of pushes, is O(1). This is the most meaningful cost metric when pushing elements that are not already in any sorted pattern.

The time complexity degrades if elements are pushed in predominantly ascending order. In the worst case, elements are pushed in ascending sorted order and the amortized cost per push is O(log(n)) against a heap containing n elements.

The worst case cost of a single call to push is O(n). The worst case occurs when capacity is exhausted and needs a resize. The resize cost has been amortized in the previous figures.

pub fn into_sorted_vec(self) -> Vec<T, A>

Consumes the BinaryHeap and returns a vector in sorted (ascending) order.

Examples

Basic usage:

use std::collections::BinaryHeap;

let mut heap = BinaryHeap::from([1, 2, 4, 5, 7]);
heap.push(6);
heap.push(3);

let vec = heap.into_sorted_vec();
assert_eq!(vec, [1, 2, 3, 4, 5, 6, 7]);

pub fn append(&mut self, other: &mut BinaryHeap<T, A>)

Moves all the elements of other into self, leaving other empty.

Examples

Basic usage:

use std::collections::BinaryHeap;

let mut a = BinaryHeap::from([-10, 1, 2, 3, 3]);
let mut b = BinaryHeap::from([-20, 5, 43]);

a.append(&mut b);

assert_eq!(a.into_sorted_vec(), [-20, -10, 1, 2, 3, 3, 5, 43]);
assert!(b.is_empty());

pub fn drain_sorted(&mut self) -> DrainSorted<'_, T, A>

🔬This is a nightly-only experimental API. (binary_heap_drain_sorted #59278)

Clears the binary heap, returning an iterator over the removed elements in heap order. If the iterator is dropped before being fully consumed, it drops the remaining elements in heap order.

The returned iterator keeps a mutable borrow on the heap to optimize its implementation.

Note:

• .drain_sorted() is O(n * log(n)); much slower than .drain(). You should use the latter for most cases.

Examples

Basic usage:

#![feature(binary_heap_drain_sorted)]
use std::collections::BinaryHeap;

let mut heap = BinaryHeap::from([1, 2, 3, 4, 5]);
assert_eq!(heap.len(), 5);

drop(heap.drain_sorted()); // removes all elements in heap order
assert_eq!(heap.len(), 0);

pub fn retain<F>(&mut self, f: F)

where F: FnMut(&T) -> bool,

Retains only the elements specified by the predicate.

In other words, remove all elements e for which f(&e) returns false. The elements are visited in unsorted (and unspecified) order.

Examples

Basic usage:

use std::collections::BinaryHeap;

let mut heap = BinaryHeap::from([-10, -5, 1, 2, 4, 13]);

heap.retain(|x| x % 2 == 0); // only keep even numbers

assert_eq!(heap.into_sorted_vec(), [-10, 2, 4])

impl<T, A> BinaryHeap<T, A>

where A: Allocator,

pub fn iter(&self) -> Iter<'_, T>

Returns an iterator visiting all values in the underlying vector, in arbitrary order.

Examples

Basic usage:

use std::collections::BinaryHeap;
let heap = BinaryHeap::from([1, 2, 3, 4]);

// Print 1, 2, 3, 4 in arbitrary order
for x in heap.iter() {
    println!("{x}");
}

pub fn into_iter_sorted(self) -> IntoIterSorted<T, A>

🔬This is a nightly-only experimental API. (binary_heap_into_iter_sorted #59278)

Returns an iterator which retrieves elements in heap order. This method consumes the original heap.

Examples

Basic usage:

#![feature(binary_heap_into_iter_sorted)]
use std::collections::BinaryHeap;
let heap = BinaryHeap::from([1, 2, 3, 4, 5]);

assert_eq!(heap.into_iter_sorted().take(2).collect::<Vec<_>>(), [5, 4]);

pub fn peek(&self) -> Option<&T>

Returns the greatest item in the binary heap, or None if it is empty.

Examples

Basic usage:

use std::collections::BinaryHeap;
let mut heap = BinaryHeap::new();
assert_eq!(heap.peek(), None);

heap.push(1);
heap.push(5);
heap.push(2);
assert_eq!(heap.peek(), Some(&5));

Time complexity

Cost is O(1) in the worst case.

pub fn capacity(&self) -> usize

Returns the number of elements the binary heap can hold without reallocating.

Examples

Basic usage:

use std::collections::BinaryHeap;
let mut heap = BinaryHeap::with_capacity(100);
assert!(heap.capacity() >= 100);
heap.push(4);

pub fn reserve_exact(&mut self, additional: usize)

Reserves the minimum capacity for at least additional elements more than the current length. Unlike reserve, this will not deliberately over-allocate to speculatively avoid frequent allocations. After calling reserve_exact, capacity will be greater than or equal to self.len() + additional. Does nothing if the capacity is already sufficient.

Panics

Panics if the new capacity overflows usize.

Examples

Basic usage:

use std::collections::BinaryHeap;
let mut heap = BinaryHeap::new();
heap.reserve_exact(100);
assert!(heap.capacity() >= 100);
heap.push(4);

pub fn reserve(&mut self, additional: usize)

Reserves capacity for at least additional elements more than the current length. The allocator may reserve more space to speculatively avoid frequent allocations. After calling reserve, capacity will be greater than or equal to self.len() + additional. Does nothing if capacity is already sufficient.

Panics

Panics if the new capacity overflows usize.

Examples

Basic usage:

use std::collections::BinaryHeap;
let mut heap = BinaryHeap::new();
heap.reserve(100);
assert!(heap.capacity() >= 100);
heap.push(4);

pub fn try_reserve_exact( &mut self, additional: usize ) -> Result<(), TryReserveError>

Tries to reserve the minimum capacity for at least additional elements more than the current length. Unlike try_reserve, this will not deliberately over-allocate to speculatively avoid frequent allocations. After calling try_reserve_exact, capacity will be greater than or equal to self.len() + additional if it returns Ok(()). Does nothing if the capacity is already sufficient.

Note that the allocator may give the collection more space than it requests. Therefore, capacity can not be relied upon to be precisely minimal. Prefer try_reserve if future insertions are expected.

Errors

If the capacity overflows, or the allocator reports a failure, then an error is returned.

Examples

use std::collections::BinaryHeap;
use std::collections::TryReserveError;

fn find_max_slow(data: &[u32]) -> Result<Option<u32>, TryReserveError> {
    let mut heap = BinaryHeap::new();

    // Pre-reserve the memory, exiting if we can't
    heap.try_reserve_exact(data.len())?;

    // Now we know this can't OOM in the middle of our complex work
    heap.extend(data.iter());

    Ok(heap.pop())
}

pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError>

Tries to reserve capacity for at least additional elements more than the current length. The allocator may reserve more space to speculatively avoid frequent allocations. After calling try_reserve, capacity will be greater than or equal to self.len() + additional if it returns Ok(()). Does nothing if capacity is already sufficient. This method preserves the contents even if an error occurs.

Errors

If the capacity overflows, or the allocator reports a failure, then an error is returned.

Examples

use std::collections::BinaryHeap;
use std::collections::TryReserveError;

fn find_max_slow(data: &[u32]) -> Result<Option<u32>, TryReserveError> {
    let mut heap = BinaryHeap::new();

    // Pre-reserve the memory, exiting if we can't
    heap.try_reserve(data.len())?;

    // Now we know this can't OOM in the middle of our complex work
    heap.extend(data.iter());

    Ok(heap.pop())
}

pub fn shrink_to_fit(&mut self)

Discards as much additional capacity as possible.

Examples

Basic usage:

use std::collections::BinaryHeap;
let mut heap: BinaryHeap<i32> = BinaryHeap::with_capacity(100);

assert!(heap.capacity() >= 100);
heap.shrink_to_fit();
assert!(heap.capacity() == 0);

pub fn shrink_to(&mut self, min_capacity: usize)

Discards capacity with a lower bound.

The capacity will remain at least as large as both the length and the supplied value.

If the current capacity is less than the lower limit, this is a no-op.

Examples

use std::collections::BinaryHeap;
let mut heap: BinaryHeap<i32> = BinaryHeap::with_capacity(100);

assert!(heap.capacity() >= 100);
heap.shrink_to(10);
assert!(heap.capacity() >= 10);

pub fn as_slice(&self) -> &[T]

🔬This is a nightly-only experimental API. (binary_heap_as_slice #83659)

Returns a slice of all values in the underlying vector, in arbitrary order.

Examples

Basic usage:

#![feature(binary_heap_as_slice)]
use std::collections::BinaryHeap;
use std::io::{self, Write};

let heap = BinaryHeap::from([1, 2, 3, 4, 5, 6, 7]);

io::sink().write(heap.as_slice()).unwrap();

pub fn into_vec(self) -> Vec<T, A>

Consumes the BinaryHeap and returns the underlying vector in arbitrary order.

Examples

Basic usage:

use std::collections::BinaryHeap;
let heap = BinaryHeap::from([1, 2, 3, 4, 5, 6, 7]);
let vec = heap.into_vec();

// Will print in some order
for x in vec {
    println!("{x}");
}

pub fn allocator(&self) -> &A

🔬This is a nightly-only experimental API. (allocator_api #32838)

Returns a reference to the underlying allocator.

pub fn len(&self) -> usize

Returns the length of the binary heap.

Examples

Basic usage:

use std::collections::BinaryHeap;
let heap = BinaryHeap::from([1, 3]);

assert_eq!(heap.len(), 2);

pub fn is_empty(&self) -> bool

Checks if the binary heap is empty.

Examples

Basic usage:

use std::collections::BinaryHeap;
let mut heap = BinaryHeap::new();

assert!(heap.is_empty());

heap.push(3);
heap.push(5);
heap.push(1);

assert!(!heap.is_empty());

pub fn drain(&mut self) -> Drain<'_, T, A>

Clears the binary heap, returning an iterator over the removed elements in arbitrary order. If the iterator is dropped before being fully consumed, it drops the remaining elements in arbitrary order.

The returned iterator keeps a mutable borrow on the heap to optimize its implementation.

Examples

Basic usage:

use std::collections::BinaryHeap;
let mut heap = BinaryHeap::from([1, 3]);

assert!(!heap.is_empty());

for x in heap.drain() {
    println!("{x}");
}

assert!(heap.is_empty());

pub fn clear(&mut self)

Drops all items from the binary heap.

Examples

Basic usage:

use std::collections::BinaryHeap;
let mut heap = BinaryHeap::from([1, 3]);

assert!(!heap.is_empty());

heap.clear();

assert!(heap.is_empty());

Trait Implementations

impl<T, A> Clone for BinaryHeap<T, A>

where T: Clone, A: Allocator + Clone,

fn clone(&self) -> BinaryHeap<T, A>

Returns a copy of the value.

fn clone_from(&mut self, source: &BinaryHeap<T, A>)

Performs copy-assignment from source.

impl<T, A> Debug for BinaryHeap<T, A>

where T: Debug, A: Allocator,

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl<T> Default for BinaryHeap<T>

where T: Ord,

fn default() -> BinaryHeap<T>

Creates an empty BinaryHeap<T>.

impl<'a, T, A> Extend<&'a T> for BinaryHeap<T, A>

where T: 'a + Ord + Copy, A: Allocator,

fn extend<I>(&mut self, iter: I)

where I: IntoIterator<Item = &'a T>,

Extends a collection with the contents of an iterator.

fn extend_one(&mut self, _: &'a T)

🔬This is a nightly-only experimental API. (extend_one #72631)

Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬This is a nightly-only experimental API. (extend_one #72631)

Reserves capacity in a collection for the given number of additional elements.

impl<T, A> Extend<T> for BinaryHeap<T, A>

where T: Ord, A: Allocator,

fn extend<I>(&mut self, iter: I)

where I: IntoIterator<Item = T>,

Extends a collection with the contents of an iterator.

fn extend_one(&mut self, item: T)

🔬This is a nightly-only experimental API. (extend_one #72631)

Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬This is a nightly-only experimental API. (extend_one #72631)

Reserves capacity in a collection for the given number of additional elements.

impl<T, const N: usize> From<[T; N]> for BinaryHeap<T>

where T: Ord,

fn from(arr: [T; N]) -> BinaryHeap<T>

use std::collections::BinaryHeap;

let mut h1 = BinaryHeap::from([1, 4, 2, 3]);
let mut h2: BinaryHeap<_> = [1, 4, 2, 3].into();
while let Some((a, b)) = h1.pop().zip(h2.pop()) {
    assert_eq!(a, b);
}

impl<T, A> From<BinaryHeap<T, A>> for Vec<T, A>

where A: Allocator,

fn from(heap: BinaryHeap<T, A>) -> Vec<T, A>

Converts a BinaryHeap<T> into a Vec<T>.

This conversion requires no data movement or allocation, and has constant time complexity.

impl<T, A> From<Vec<T, A>> for BinaryHeap<T, A>

where T: Ord, A: Allocator,

fn from(vec: Vec<T, A>) -> BinaryHeap<T, A>

Converts a Vec<T> into a BinaryHeap<T>.

This conversion happens in-place, and has O(n) time complexity.

impl<T> FromIterator<T> for BinaryHeap<T>

where T: Ord,

fn from_iter<I>(iter: I) -> BinaryHeap<T>

where I: IntoIterator<Item = T>,

Creates a value from an iterator.

impl<'a, T, A> IntoIterator for &'a BinaryHeap<T, A>

where A: Allocator,

type Item = &'a T

The type of the elements being iterated over.

type IntoIter = Iter<'a, T>

Which kind of iterator are we turning this into?

fn into_iter(self) -> Iter<'a, T>

Creates an iterator from a value.

impl<T, A> IntoIterator for BinaryHeap<T, A>

where A: Allocator,

fn into_iter(self) -> IntoIter<T, A>

Creates a consuming iterator, that is, one that moves each value out of the binary heap in arbitrary order. The binary heap cannot be used after calling this.

Examples

Basic usage:

use std::collections::BinaryHeap;
let heap = BinaryHeap::from([1, 2, 3, 4]);

// Print 1, 2, 3, 4 in arbitrary order
for x in heap.into_iter() {
    // x has type i32, not &i32
    println!("{x}");
}

type Item = T

The type of the elements being iterated over.

type IntoIter = IntoIter<T, A>

Which kind of iterator are we turning this into?