104.二叉树的最大深度。2星

方法一：

/**
 * 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;}
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        int maxceng=0;
        
        queue.offer(root);
        queue.offer(null);
        //将一层的节点全部入栈后，加入一个空节点作为标志位
        while(!queue.isEmpty()){
            TreeNode st2 = queue.poll();
            if(st2==null){
                maxceng++;
                if(!queue.isEmpty()) queue.offer(null);
                //防止在遍历完成后多次加入空节点造成死循环
            }
            else {
                if (st2.left!=null){queue.offer(st2.left);}
                if (st2.right!=null){queue.offer(st2.right);}
            }
        }
        return maxceng;
    }
}

方法二：
递归前序遍历：

/**
 * 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;
        }
        
        int left = maxDepth(root.left);
        int right = maxDepth(root.right);

        return left > right ? left + 1 : right + 1;
    }
}