递归-leetcode104. Maximum Depth of Binary Tree

一、问题描述
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.
Note: A leaf is a node with no children.
Example:
Given binary tree [3,9,20,null,null,15,7],
3
/
9 20
/
15 7
return its depth = 3.

二、解决思路
思路一：采用递归来解决，二叉树的高度等于左右子树高度最大值 + 1

三、算法实现

public int maxDepth(TreeNode root) {
        if(root == null) return 0;
        int left = maxDepth(root.left);
        int right = maxDepth(root.right);
        return Math.max(left, right) + 1;
    }