Add unsafe constructor BinaryHeap::from_vec_cmp_raw() (sekineh#22)

Author: davidli2010

CHANGELOG.md

@@ -6,6 +6,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+* generic constructor `from_vec_cmp_raw()`.
+
 ## [0.3.0] - 2020-07-08
 
 ### Added

src/binary_heap.rs

@@ -394,8 +394,21 @@ impl<T, C: Compare<T>> BinaryHeap<T, C> {
     /// Because `BinaryHeap` stores the elements in its internal `Vec`,
     /// it's natural to construct it from `Vec`.
     pub fn from_vec_cmp(vec: Vec<T>, cmp: C) -> Self {
+        unsafe { BinaryHeap::from_vec_cmp_raw(vec, cmp, true) }
+    }
+
+    /// Generic constructor for `BinaryHeap` from `Vec` and comparator.
+    ///
+    /// Because `BinaryHeap` stores the elements in its internal `Vec`,
+    /// it's natural to construct it from `Vec`.
+    ///
+    /// # Safety
+    /// User is responsible for providing valid `rebuild` value.
+    pub unsafe fn from_vec_cmp_raw(vec: Vec<T>, cmp: C, rebuild: bool) -> Self {
         let mut heap = BinaryHeap { data: vec, cmp };
-        heap.rebuild();
+        if rebuild && !heap.data.is_empty() {
+            heap.rebuild();
+        }
         heap
     }
 }