Leetcode——104. Maximum Depth of Binary Tree

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本文介绍了解决LeetCode上二叉树最大深度问题的两种方法：递归法和层级遍历法。递归法通过左右子树的最大深度加一得出结果；层级遍历法则利用队列进行逐层节点数的统计。

题目原址

https://leetcode.com/problems/maximum-depth-of-binary-tree/description/

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],

return its depth = 3.

AC代码

示例一：使用递归

class Solution {
    public int maxDepth(TreeNode root) {
        if(root == null)
            return 0;

        return Math.max( maxDepth(root.left), maxDepth(root.right)) + 1;          
    }
}

示例二：使用层的方法，与题637题一样。Leetcode——637. Average of Levels in Binary Tree

class Solution {
    public int maxDepth(TreeNode root) {
        int depth = 0;
        if(root == null)
            return depth;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while(queue.size() > 0) {
            int size = queue.size();
            depth ++;
            for(int i = 0; i < size; i++) {
                TreeNode tn = queue.poll();
                if(tn.left != null)
                    queue.add(tn.left);
                if(tn.right != null)
                    queue.add(tn.right);
            }
        }
        return depth;
    }
}