递归实现

/*** Definition for a binary tree node.* public class TreeNode {*     int val;*     TreeNode left;*     TreeNode right;*     TreeNode() {}*     TreeNode(int val) { this.val = val; }*     TreeNode(int val, TreeNode left, TreeNode right) {*         this.val = val;*         this.left = left;*         this.right = right;*     }* }*/
class Solution {public int maxDepth(TreeNode root) {if(root==null){return 0;}else{return Math.max(maxDepth(root.left),maxDepth(root.right))+1;}}
}

队列实现，遍历每一行元素后dept++

/*** Definition for a binary tree node.* public class TreeNode {*     int val;*     TreeNode left;*     TreeNode right;*     TreeNode() {}*     TreeNode(int val) { this.val = val; }*     TreeNode(int val, TreeNode left, TreeNode right) {*         this.val = val;*         this.left = left;*         this.right = right;*     }* }*/
class Solution {public int maxDepth(TreeNode root) {if(root==null){return 0;}Queue<TreeNode> queue = new LinkedList<TreeNode>();queue.offer(root);int dept=0;while(!queue.isEmpty()){int size=queue.size();while(size>0){TreeNode node=queue.poll();if(node.left!=null){queue.offer(node.left);}if(node.right!=null){queue.offer(node.right);}size--;}dept++;}return dept;}
}