Add solution #508

Author: JoshCrozier

README.md

@@ -407,6 +407,7 @@
 504|[Base 7](./0504-base-7.js)|Easy|
 506|[Relative Ranks](./0506-relative-ranks.js)|Easy|
 507|[Perfect Number](./0507-perfect-number.js)|Easy|
+508|[Most Frequent Subtree Sum](./0508-most-frequent-subtree-sum.js)|Medium|
 509|[Fibonacci Number](./0509-fibonacci-number.js)|Easy|
 520|[Detect Capital](./0520-detect-capital.js)|Easy|
 521|[Longest Uncommon Subsequence I](./0521-longest-uncommon-subsequence-i.js)|Easy|

solutions/0508-most-frequent-subtree-sum.js

@@ -0,0 +1,51 @@
+/**
+ * 508. Most Frequent Subtree Sum
+ * https://leetcode.com/problems/most-frequent-subtree-sum/
+ * Difficulty: Medium
+ *
+ * Given the root of a binary tree, return the most frequent subtree sum. If there is a tie, return
+ * all the values with the highest frequency in any order.
+ *
+ * The subtree sum of a node is defined as the sum of all the node values formed by the subtree
+ * rooted at that node (including the node itself).
+ */
+
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val, left, right) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.left = (left===undefined ? null : left)
+ *     this.right = (right===undefined ? null : right)
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @return {number[]}
+ */
+var findFrequentTreeSum = function(root) {
+  if (!root) return [];
+
+  const map = new Map();
+  let result = [];
+  let max = 0;
+
+  traverse(root);
+
+  for (const [sum, freq] of map) {
+    if (freq > max) {
+      max = freq;
+      result = [sum];
+    } else if (freq === max) {
+      result.push(sum);
+    }
+  }
+
+  return result;
+
+  function traverse(node) {
+    if (!node) return 0;
+    const sum = node.val + traverse(node.left) + traverse(node.right);
+    map.set(sum, (map.get(sum) || 0) + 1);
+    return sum;
+  }
+};