LeetCode Hot100:递归穿透值传递问题

题目：104.二叉树的最大深度

1.自底向上依靠return

2.自顶向下依靠全局变量接收

//1.自底向上
/*** 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 maxDepth = function (root) {function dfs(root) {if (!root) {return 0;}leftDepth = dfs(root.left);rightDepth = dfs(root.right);return Math.max(leftDepth, rightDepth) + 1;}return dfs(root);
};//自顶向下
/*** 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 maxDepth = function (root) {let depth = 0;let ans = 0;function dfs(root, depth) {if (!root) {return;}depth++;ans = Math.max(ans, depth);dfs(root.left, depth);dfs(root.right, depth);}dfs(root, depth);return ans;
};