【一次过】Lintcode 70. 二叉树的层次遍历 II

 

给出一棵二叉树，返回其节点值从底向上的层次序遍历（按从叶节点所在层到根节点所在的层遍历，然后逐层从左往右遍历）

样例

给出一棵二叉树 {3,9,20,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

按照从下往上的层次遍历为：

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

解题思路：

    与Lintcode 69:Binary Tree Level Order Traversal思路一致，只是逆序输出，所以用stack暂存，最后取出即可。

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */

public class Solution {
    /**
     * @param root: A tree
     * @return: buttom-up level order a list of lists of integer
     */
    public List> levelOrderBottom(TreeNode root) {
        // write your code here
        List> res = new ArrayList<>();
        if(root == null)
            return res;
        LinkedList queue = new LinkedList<>();
        queue.offer(root);
        Stack> stack = new Stack<>();
        
        while(!queue.isEmpty()){
            int len = queue.size();
            List temp = new ArrayList<>();
            
            for(int i=0; i