题解 | #二叉树的最大深度#TOP28

思路:
1.递归 Math.max(getMaxHeight(root.left) + 1, getMaxHeight(root.right)+1)
2.记得之前的TOP26层序遍历吗？有多少层深度就是多少，所以直接采用队列，看有多少层就ok了

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
        
        //return getMaxHeight(root);
        return maxHeightDFS(root);
    }
    private int getMaxHeight(TreeNode root){
        if(root == null){
            return 0;
        }
        int left = getMaxHeight(root.left) + 1;
        int right = getMaxHeight(root.right) + 1;
        return Math.max(left, right);
    }
    private int maxHeightDFS(TreeNode root){
        if(root == null){
            return 0;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        int res = 0;
        queue.add(root);
        while(!queue.isEmpty()){
            int size = queue.size();
            while(size -- > 0){
                TreeNode node = queue.poll();
                if(node.left != null){
                    queue.add(node.left);
                }
                if(node.right != null){
                    queue.add(node.right);
                }
            }
            res ++ ;
        }
        return res;
    }
}