107.二叉树的层次遍历-ii.java

import java.util.LinkedList;
import java.util.Queue;

 

/*
 * @lc app=leetcode.cn id=107 lang=java
 *
 * [107] 二叉树的层次遍历 II
 */
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        List<List<Integer>> warpList = new LinkedList<>();

        if(root == null) return warpList;

        //当队列为空时 使用add会出错 与add属于不同接口的方法
        queue.offer(root);
        while(!queue.isEmpty()){

            int levelNum = queue.size();
            List<Integer> subList = new LinkedList<>();
            for(int i=0; i<levelNum; i++){
                if(queue.peek().left != null) queue.offer(queue.peek().left);
                if(queue.peek().right != null) queue.offer(queue.peek().right);
                subList.add(queue.poll().val);
            }
        //向上还是向下没关系 处理数组的时候加个技巧就ok
        warpList.add(0,subList);
        }
        return warpList;
    }

}