Leetcode題解 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;
    }
}