牛客-TOP101-BM28

核心思路：层序遍历+Deque

import java.util.*;

/* * public class TreeNode { * int val = 0; * TreeNode left = null; * TreeNode right = null; * } */

public class Solution {
    
    /** * * @param root TreeNode类 * @return int整型 */
    public int maxDepth (TreeNode root) {
    
        // write code here
        if(root == null){
    
            return 0;
        }
        Deque<TreeNode> deque = new ArrayDeque<>();
        deque.addLast(root);
        int count = 0;
        while(!deque.isEmpty()){
    
            int n = deque.size();
            for(int i = 0; i < n; i++){
    
                TreeNode node = deque.pollFirst();
                if(node.left != null)
                    deque.offerLast(node.left);
                if(node.right != null)
                    deque.offerLast(node.right);
            }
            count++;
        }
        return count;
    }
}

public class Solution {
    
    /** * * @param root TreeNode类 * @return int整型 */
    public int maxDepth (TreeNode root) {
    
        if(root == null){
    
            return 0;
        }
        return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
    }
}