Leetcode题解 107. Binary Tree Level Order Traversal II

Given a binary tree, return the bottom-up level order traversal of its nodes’ values. (ie, from left to right, level by level from leaf to root).

For example:
Given binary tree [3,9,20,null,null,15,7],

    3   / \  9  20    /  \   15   7

return its bottom-up level order traversal as:

[  [15,7],  [9,20],  [3]]

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public List<List<Integer>> levelOrderBottom(TreeNode root) {        List<List<Integer>> result =new ArrayList<List<Integer>>();        List<TreeNode> levelResult=new ArrayList<TreeNode>();        if(root==null)  return result;        levelResult.add(root);        List<Integer> first=new ArrayList<Integer>();        first.add(root.val);        result.add(first);        while(levelResult.size()>0){            List<TreeNode> tempResult=new ArrayList<TreeNode>();            List<Integer> everyLevel=new ArrayList<Integer>();            for(TreeNode temp:levelResult){                if(temp.left!=null){                    tempResult.add(temp.left);                    everyLevel.add(temp.left.val);                }                if(temp.right!=null){                    tempResult.add(temp.right);                    everyLevel.add(temp.right.val);                }            }            if(everyLevel.size()>0)                result.add(0,everyLevel);            levelResult=tempResult;        }        return result;    }}