1/**
2 * Definition for a binary tree node.
3 * function TreeNode(val, left, right) {
4 * this.val = (val===undefined ? 0 : val)
5 * this.left = (left===undefined ? null : left)
6 * this.right = (right===undefined ? null : right)
7 * }
8 */
9/**
10 * @param {TreeNode} root
11 * @return {number}
12 */
13var diameterOfBinaryTree = function (root) {
14 if (root === null || (root.left === null && root.right === null)) return 0
15 let res = 0
16 function dfs(root) {
17 if (root === null) return 0
18 let left = dfs(root.left)
19 let right = dfs(root.right)
20 res = Math.max(res, left + right + 1)
21 return Math.max(left, right) + 1
22 }
23 dfs(root)
24 return res - 1
25};