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;
};