【leetcode】107. Binary Tree Level Order Traversal II

一、题目描述

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]]

题目解读：对一棵树进行层次遍历，每一层的节点在一个列表中，所有层的节点列表的逆序组成一个总列表，返回总列表

思路：进行层次遍历即可

c++代码（8ms，22.99%）

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector<vector<int>> result;        void levelTra(TreeNode *root, int level){        if(root == NULL)            return;        if(level == result.size()){            vector<int> v;            result.push_back(v);        }        result[level].push_back(root->val);        levelTra(root->left, level+1);        levelTra(root->right, level+1);    }    vector<vector<int>> levelOrderBottom(TreeNode* root) {        levelTra(root, 0);        return vector<vector<int>> (result.rbegin(), result.rend());    }};