Maximum Depth of Binary Tree

写法仿照Find Leaves of Binary Tree的DFS

注意一下 怎么不用递归写DFS

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