binary-tree-level-order-traversal-ii

题目：

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,#,#,15,7},
3
/ \
9 20
/ \
15 7

return its bottom-up level order traversal as:
[
[15,7]
[9,20],
[3],
]

程序：

class Solution {public:    vector<vector<int> > levelOrderBottom(TreeNode *root) {        queue<TreeNode*> q;        vector<vector<int>> vv;        if (root == NULL){            return vv;        }        int before = 0;        int current = 0;        if (root != NULL && q.empty()){            q.push(root);            current++;        }        while (!q.empty()){            vector<int>v;            before = current;            current = 0;            while(before--){                TreeNode *t = q.front();                if (t->left != NULL){                    q.push(t->left);                    current++;                }                if (t->right != NULL){                    q.push(t->right);                    current++;                }                v.push_back(t->val);                q.pop();            }            vv.push_back(v);        }        vector<vector<int>> res;        for (int i = vv.size()-1; i >= 0; --i){            res.push_back(vv[i]);        }        return res;    }};