adding another leetcode pq problem

lectures/22_priority_queues/README.md

@@ -125,5 +125,4 @@ Lambda is new to the C++ language (part of C++11). But lambda is a core piece of
 
 - [Leetcode problem 215: Kth Largest Element in an Array](https://leetcode.com/problems/kth-largest-element-in-an-array/). Solution: [p215_kth_largest_element.cpp](../../leetcode/p215_kth_largest_element.cpp).
 - [Leetcode problem 373: Find K Pairs with Smallest Sums](https://leetcode.com/problems/find-k-pairs-with-smallest-sums/). Solution: [p373_k_pairs_smallest_sums.cpp](../../leetcode/p373_k_pairs_smallest_sums.cpp).
-- [Leetcode problem 692:  Top K Frequent Words](https://leetcode.com/problems/implement-stack-using-queues/). Solution: [p225_stack_using_queues.cpp](../../leetcode/p225_stack_using_queues.cpp).
+- [Leetcode problem 692:  Top K Frequent Words](https://leetcode.com/problems/top-k-frequent-words/). Solution: [p692_top_k_frequent_words.cpp](../../leetcode/p692_top_k_frequent_words.cpp).
-- [Leetcode problem 912: Sort an Array](https://leetcode.com/problems/implement-queue-using-stacks/). Solution: [p232_queue_using_stacks.cpp](../../leetcode/p232_queue_using_stacks.cpp).

leetcode/p692_top_k_frequent_words.cpp

@@ -0,0 +1,50 @@
+class cmp{
+public:
+    // this is how we overload the function call operator, and this has to a public method.
+    bool operator() (std::pair<string,int> p1, std::pair<string,int> p2){
+        if(p1.second==p2.second){
+            // lexicographical order
+            return (p1.first < p2.first);
+        }else{
+            // sort this way would make sure the top of the heap is the string with the lowest frequency.
+            return (p1.second > p2.second);
+        }
+    }
+};
+
+class Solution {
+public:
+    vector<string> topKFrequent(vector<string>& words, int k) {
+        int size = words.size();
+        // map words to frequency
+        unordered_map<string, int> u_map1;
+        vector<string> v1;
+        // establish the map
+        for(int i=0;i<size;i++){
+            u_map1[words[i]]++;
+        }
+        // define a priority queue (MAX heap) with a custom comparison function
+        priority_queue<pair<string, int>,vector<pair<string,int>>,cmp> pq;
+        // traverse the map so as to establish the priority queue.
+        unordered_map<string, int>::iterator itr;
+        itr = u_map1.begin();
+        while(itr != u_map1.end()){
+            // push each pair into u_map1.
+            pq.push(*itr);
+            if(pq.size()>k){
+                // evict the string with the lowest frequency?
+                pq.pop();
+            }
+            itr++;
+        }
+        // after the above for loop, pq will have a size of k, and contains the strings with the highest frequency.
+        // now just move these strings into v1.
+        for(int i=0;i<k;i++){
+            // we have to use top() here, beause pop() won't return anything.
+            v1.push_back((pq.top()).first);
+            pq.pop();
+        }
+        std:reverse(v1.begin(),v1.end());
+        return v1;
+    }
+};