LeetCode（六）104. Maximum Depth of Binary Tree

题目描述：
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.

思路说明：采用深度优先搜索的方式，不断的递归找出左子树的深度和右子树的深度，最后选择深度最大的那个长度

代码实现：

class Solution {
	public class TreeNode {
		int val;
		TreeNode left;
		TreeNode right;

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