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leetCode-107:Binary Tree Level Order Traversal II

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问题描述

给定一个二叉树，要求从下到上，从左到右进行遍历输出。题目链接：**点我**

样例输入输出

输入：[1,2,3,4,5]

输出：[[4,5],[2,3],[1]]

解释：树的结构如下

1

/ \

2 3

/\

4 5

输入：[1]

输出：[[1]]

问题解法

用广度优先搜索算法进行遍历，在遍历每一层后，把数据直接放到链表头部。代码如下

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution
{
    public List<List<Integer>> levelOrderBottom(TreeNode root)
    {
        Queue<TreeNode> queue = new LinkedList<>();
        List<List<Integer>> result = new LinkedList<>();
        if (root == null)
        {
            return result;
        }
        
        queue.offer(root);
        while (!queue.isEmpty())
        {
            List<Integer> level = new LinkedList<>();
            for (int i = queue.size(); i > 0; i--)
            {
                TreeNode node = queue.poll();
                level.add(node.val);
                if (node.left != null)
                {
                    queue.offer(node.left);
                }

                if (node.right != null)
                {
                    queue.offer(node.right);
                }
            }

            result.add(0, level);
        }

        return result;
    }
}