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;
    }
};