Java解决：递归和常规解法

方法1：简单递归方式

public class TreeNode{
    public int maxDepth(TreeNode root){
        if(root==null)return 0;
        int leftDepth = maxDepth(root.left);
        int rightDepth = maxDepth(root.right);
        int res = Math.max(leftDepth,rightDepth);
        return res+1;
    }
}

方法2：通过链表

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
     */
    //递归
    /*
    public int maxDepth (TreeNode root) {
        if(root == null)return 0;
        int leftDepth = maxDepth(root.left);
        int rightDepth = maxDepth(root.right);
        return leftDepth > rightDepth ? leftDepth + 1 : rightDepth + 1;
        // write code here
    }
    */
    public int maxDepth(TreeNode root){ 
        if(root == null)return 0;
        LinkedList<TreeNode> list = new LinkedList<>();
        list.offer(root);
        int max = 0;
        int level = 0;
        int size = 0;
        int cur = 0;
        while(!list.isEmpty()){
            size = list.size();
            max = Math.max(max,size);
            cur = 0;
            while(cur < size){
                TreeNode node = list.poll();
                cur++;
                if(node.left != null){
                    list.offer(node.left);
                }
                if(node.right != null){
                    list.offer(node.right);
                }
            }
            level++;
        }
        //System.out.println("二叉树最大宽度为："+max)；
        return level;
    }
}