Given a binary tree, find its maximum depth.
The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.
Tags: Tree, Depth-first Search
题意是找到二叉树的最大深度，很明显，深搜即可，每深入一次节点加一即可，然后取左右子树的最大深度。
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public int maxDepth(TreeNode root) {
if (root == null) return 0;
return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
}
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