leetcode 104 maximum-depth-of-binary-tree

https://leetcode.cn/problems/maximum-depth-of-binary-tree/submissions/
https://www.bilibili.com/video/BV11Y4y1q7YA?p=27

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

来源：力扣（LeetCode）

/**
 * 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<>();
        queue.offer(root);
        int depth = 0;
        while(!queue.isEmpty()){
            /*size is used to check if those node at the same level 
                have been completely processed 
            */
            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--;
            }
            depth++;
        }
        return depth;
    }
    // public int maxDepth(TreeNode root) {
    //     if(root == null){
    //         return 0;
    //     }else{
    //         return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
    //     }
    // }
}