Create most-frequent-subtree-sum.cpp

Author: kamyu104

C++/most-frequent-subtree-sum.cpp

@@ -0,0 +1,41 @@
+// Time:  O(n)
+// Space: O(n)
+
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+ * };
+ */
+class Solution {
+public:
+    vector<int> findFrequentTreeSum(TreeNode* root) {
+        unordered_map<int,int> counts;
+        int max_count = 0;
+        countSubtreeSumsHelper(root, &counts, &max_count);
+        
+        vector<int> result;
+        for (const auto& kvp : counts){
+            if (kvp.second == max_count) {
+                result.emplace_back(kvp.first);
+            }
+        }
+        return result;
+    }
+
+private:
+    int countSubtreeSumsHelper(TreeNode *root, unordered_map<int,int> *counts, int *max_count) {
+        if (!root) {
+            return 0;
+        }
+        auto sum = root->val +
+                   countSubtreeSumsHelper(root->left, counts, max_count) +
+                   countSubtreeSumsHelper(root->right, counts, max_count);
+        ++(*counts)[sum];
+        (*max_count) = max((*max_count), (*counts)[sum]);
+        return sum;
+    }
+};