二叉树-二叉树的层次遍历-中等

描述

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）

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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]
]

题目链接

程序

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

class Solution {
public:
    /**
     * @param root: A tree
     * @return: buttom-up level order a list of lists of integer
     */
    vector<vector<int>> levelOrderBottom(TreeNode * root) {
        // write your code here
        vector<vector<int>> result;
        if(root == NULL)
            return result;
        std::queue<TreeNode *> Q;
        Q.push(root);//创造仅含根节点的队列
        while(!Q.empty()){//如果队列为空，则结束
            vector<int> level;
            int size = Q.size();
            
            for(int i = 0; i < size; i++){
                TreeNode *head = Q.front();
                Q.pop();//取队列顶的元素，并弹出
                level.push_back(head->val);
                if(head->left != NULL)//将该节点所有的孩子节点压入队列
                    Q.push(head->left);
                if(head->right != NULL)
                    Q.push(head->right);
            }
            
            result.push_back(level);
        }
        reverse(result.begin(),result.end());
        return result;
    }
};