NC13题解 | #二叉树的最大深度#

• 解法1：递归dfs
• 解法2：队列bfs层序遍历

解法1：递归

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;
        }
        int res = dfs(root,0);
        return res;
        
    }
    //递归dfs遍历
    public int dfs(TreeNode root, int depth){
        if(root == null) {
            return depth;
        }
        if(root.left == null && root.right == null){
            return depth+1;
        }
        return Math.max(dfs(root.left,depth+1),dfs(root.right,depth+1));
    }
}

解法2：队列bfs层序遍历

public class Solution {
    /**
     * 
     * @param root TreeNode类 
     * @return int整型
     */
    public int maxDepth (TreeNode root) {
        // write code here
        if(root == null) {
            return 0;
        }
        int res = bfs(root);
        return res;
        
    }
 	//队列层序遍历
    public int bfs(TreeNode root){
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        int res = 0;
        while(!queue.isEmpty()){
            int size = queue.size();
            for(int i = 0; i < size; i++){
                TreeNode node = queue.poll();
                //list放入null会占用一个空间，
                //list加入null判断isEmpty结果是否，size同样会加1
                if(node.left != null){
                    queue.add(node.left);
                }
                if(node.right != null){
                    queue.add(node.right);
                }
            }
            res++;
        }
        return res;
    }
}