Add solution #687

Author: JoshCrozier

README.md

@@ -518,6 +518,7 @@
 684|[Redundant Connection](./0684-redundant-connection.js)|Medium|
 685|[Redundant Connection II](./0685-redundant-connection-ii.js)|Hard|
 686|[Repeated String Match](./0686-repeated-string-match.js)|Easy|
+687|[Longest Univalue Path](./0687-longest-univalue-path.js)|Medium|
 693|[Binary Number with Alternating Bits](./0693-binary-number-with-alternating-bits.js)|Easy|
 695|[Max Area of Island](./0695-max-area-of-island.js)|Medium|
 696|[Count Binary Substrings](./0696-count-binary-substrings.js)|Easy|

solutions/0687-longest-univalue-path.js

@@ -0,0 +1,40 @@
+/**
+ * 687. Longest Univalue Path
+ * https://leetcode.com/problems/longest-univalue-path/
+ * Difficulty: Medium
+ *
+ * Given the root of a binary tree, return the length of the longest path, where each node
+ * in the path has the same value. This path may or may not pass through the root.
+ *
+ * The length of the path between two nodes is represented by the number of edges between them.
+ */
+
+/**
+ * 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 longestUnivaluePath = function(root) {
+  let max = 0;
+  dfs(root);
+  return max;
+
+  function dfs(node) {
+    if (!node) return 0;
+
+    const left = dfs(node.left);
+    const right = dfs(node.right);
+    const leftPath = node.left?.val === node.val ? left + 1 : 0;
+    const rightPath = node.right?.val === node.val ? right + 1 : 0;
+
+    max = Math.max(max, leftPath + rightPath);
+    return Math.max(leftPath, rightPath);
+  }
+};