BinaryHeap: add min-heap example

src/liballoc/collections/binary_heap.rs

@@ -207,6 +207,30 @@ use super::SpecExtend;
 /// // The heap should now be empty.
 /// assert!(heap.is_empty())
 /// ```
+///
+/// ## Min-heap
+///
+/// Either `std::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);
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct BinaryHeap<T> {
     data: Vec<T>,