adding functor solution

lectures/23_priority_queues_II/README.md

@@ -122,5 +122,8 @@ organized heap data, but incur a O(n log n) cost. Why?
 
 ## 23.10 Leetcode Exercises
 
-- [Leetcode problem 912: Sort an Array](https://leetcode.com/problems/sort-an-array/). Solution: [p912_heapsort_array.cpp](../../leetcode/p912_heapsort_array.cpp).
-Solution 2: [p912_heapsort_array_heapify.cpp](../../leetcode/p912_heapsort_array_heapify.cpp). Solution 3 - min heap: [p912_heapsort_array_min_heap.cpp](../../leetcode/p912_heapsort_array_min_heap.cpp).
+- [Leetcode problem 912: Sort an Array](https://leetcode.com/problems/sort-an-array/).  
+  - Solution: [p912_heapsort_array.cpp](../../leetcode/p912_heapsort_array.cpp).  
+  - Solution 2: [p912_heapsort_array_heapify.cpp](../../leetcode/p912_heapsort_array_heapify.cpp).  
+  - Solution 3: - min heap: [p912_heapsort_array_min_heap.cpp](../../leetcode/p912_heapsort_array_min_heap.cpp).  
+  - Solution 4: - min heap, with functor: [p912_heapsort_array_functor.cpp](../../leetcode/p912_heapsort_array_functor.cpp).

leetcode/p912_heapsort_array_functor.cpp

@@ -0,0 +1,27 @@
+class cmp{
+public:
+    // this is how we overload the function call operator, and this has to a public method.
+    bool operator() (int A, int B){
+        return (A > B);
+    }
+};
+
+class Solution {
+public:
+    vector<int> sortArray(vector<int>& nums) {
+        vector<int> result;
+        priority_queue<int, vector<int>, cmp> pq;
+        int size = nums.size();
+        // build the priority queue, which is now a min heap
+        for(int i=0;i<size;i++){
+            pq.push(nums[i]);
+        }
+	// pop out the smallest element size times
+        for(int i=0;i<size;i++){
+            result.push_back(pq.top());
+            pq.pop();
+        }
+        // now data is sorted in an ascending order
+        return result;
+    }
+};