104. Maximum Depth of Binary Tree

难度: Easy

题目描述

Given the root of a binary tree, return its maximum depth.

A binary tree's maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

 

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: 3

Example 2:

Input: root = [1,null,2]
Output: 2

 

Constraints:

• The number of nodes in the tree is in the range [0, 104].
• -100 <= Node.val <= 100

解题思路

代码实现

解决方案

java

/**
 * 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;
        }
        return Math.max(maxDepthLevelTravel(root.left), maxDepthLevelTravel(root.right)) + 1;
    }

    public int maxDepthLevelTravel(TreeNode root) {
        if (root == null) {
            return 0;
        }

        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        int maxDepth = 0;
        while (!queue.isEmpty()) {
            int size = queue.size();
            maxDepth++;

            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                if (node != null) {
                    if (node.left != null) {
                        queue.offer(node.left);
                    }

                    if (node.right != null) {
                        queue.offer(node.right);
                    }

                }
            }
        }
        return maxDepth;
    }


}