LeetCode刷题（C++）——Binary Tree Level Order Traversal II（Easy）

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. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector<vector<int>> levelOrderBottom(TreeNode* root) {        vector<vector<int>> levels;if (root == NULL)return levels;vector<int> onelevel;queue<TreeNode*> q;q.push(root);int nextlevel = 0;int curlevel = 1;while (!q.empty()){TreeNode* p = q.front();onelevel.push_back(p->val);if (p->left){q.push(p->left);++nextlevel;}if (p->right){q.push(p->right);++nextlevel;}q.pop();--curlevel;if (curlevel == 0){levels.insert(levels.begin(), onelevel);curlevel = nextlevel;nextlevel = 0;onelevel.clear();}}return levels;    }};