adding one problem using both unordered map and tree

leetcode/p508_most_frequent_subtree_sum.cpp

@@ -0,0 +1,56 @@
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+class Solution {
+public:
+    // get the subtree sum, update map1, and also update the maxFreq
+    int helper(TreeNode* node, std::unordered_map<int, int>& map1, int& maxFreq){
+        int sum;
+        int freq;
+        // base case
+        if(node==nullptr){
+            return 0;
+        }else if(node->left==nullptr && node->right==nullptr){
+            freq = (++map1[node->val]);
+            if(freq>maxFreq){
+                maxFreq = freq;
+            }
+            return node->val;
+        }
+        // general case
+        sum = node->val + helper(node->left, map1, maxFreq) + helper(node->right, map1, maxFreq);
+        freq = (++map1[sum]);
+        if(freq>maxFreq){
+            maxFreq = freq;
+        }
+        return sum;
+    }
+
+    vector<int> findFrequentTreeSum(TreeNode* root) {
+        int sum = 0;
+        int maxFreq = 0;
+        vector<int> result;
+        // sum and frequency
+        std::unordered_map<int, int> map1;
+        // find the tree sum of each subtree
+        if(root!=nullptr){
+            sum = helper(root, map1, maxFreq);
+        }
+        std::unordered_map<int, int>::iterator itr1 = map1.begin();
+        while(itr1!=map1.end()){
+            if(itr1->second == maxFreq){
+                result.push_back(itr1->first);
+            }
+            itr1++;
+        }
+        return result;
+    }
+};