104. Maximum Depth of Binary Tree (Easy)

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.

题目即求二叉树的高度，层序遍历二叉树求解即可。

Solution：

Java：

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public int maxDepth(TreeNode root) {        if(root == null) {            return 0;        }        if(root.left == null && root.right == null) {            return 1;        }        int maxDepth = 0;        Queue<TreeNode> queue = new LinkedList<>();        queue.add(root);        int currentLevelCount = 1;        int nextLevelCount = 0;        while(!queue.isEmpty()) {            TreeNode node = queue.poll();            currentLevelCount--;            if(node.left != null) {                queue.add(node.left);                nextLevelCount++;            }            if(node.right != null) {                queue.add(node.right);                nextLevelCount++;            }            if(currentLevelCount == 0) {                currentLevelCount = nextLevelCount;                nextLevelCount = 0;                maxDepth++;            }        }        return maxDepth;    }}

非递归的层序遍历，通过队列来实现，使用currentLevelCount和nextLevelCount两个变量记录当前层的结点个数和下一层的结点个数。