added JS Most Frequent Subtree Sum

js/most-frequent-subtree-sum.js

@@ -0,0 +1,55 @@
+// https://leetcode.com/problems/most-frequent-subtree-sum/
+// Related Topics: Hash Table, Tree, DFS
+// Difficulty: Medium
+
+/*
+Initial thoughts:
+Traversing the tree in a post order DFS manner, we are going to create
+a freqency table of all the subtree sums and return those sums that occur
+most at the end.
+
+Time Complexity: O(n) where n == number of nodes in the tree
+Space Complexity: O(n) where n == number of nodes in the tree (In a worst case
+situation, each and every subtree has a unique sum that requires a separate entry
+in our freqency table)
+*/
+
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @return {number[]}
+ */
+const findFrequentTreeSum = root => {
+    if (!root) return [];
+
+    const m = new Map();
+
+    function dfs(root) {
+        if (!root) return 0;
+
+        const left = dfs(root.left);
+        const right = dfs(root.right);
+
+        const val = root.val + left + right;
+        if (m.has(val)) m.set(val, m.get(val) + 1);
+        else m.set(val, 1);
+        return val;
+    }
+
+    dfs(root);
+    let max = Number.NEGATIVE_INFINITY;
+    let finalRes;
+    for (let [key, val] of m.entries()) {
+        if (val > max) {
+            max = val;
+            finalRes = [key];
+        } else if (val === max) finalRes.push(key);
+    }
+    return finalRes;
+};