Merge pull request #440 from UberschallSamsara/AddIsHeap

Add is_heap.md for upstream repo issue #31

algorithm/is_heap.md

@@ -0,0 +1,31 @@
+# is_heap
+
+**Description :** The C++ function `std::is_heap` returns `true` if the elements in the range `[first, last)` form a _max heap_, such as is constructed by _make_heap_, and `false` otherwise.
+
+**Example :**
+
+```cpp
+#include <algorithm>
+#include <iostream>
+#include <vector>
+
+int main()
+{
+    std::vector<int> v { 8, 6, 7, 5, 3, 0, 9 };
+
+    std::cout << "Intial value for v: ";
+    for (auto i : v) std::cout << i << ' ';
+    std::cout << '\n';
+
+    if (!std::is_heap(v.begin(), v.end())) {
+        std::cout << "Creating heap:\n";
+        std::make_heap(v.begin(), v.end());
+    }
+
+    std::cout << "After call to make_heap, v: ";
+    for (auto i : v) std::cout << i << ' ';
+    std::cout << '\n';
+}
+```
+
+**[Run Code](https://rextester.com/CWLO88991)**