leetcode 107. Binary Tree Level Order Traversal II

题目描述：

//利用队列 进行广度优先搜索

/**
 * 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) {
        //广度优先搜索之后逆序输出 用队列
        vector<vector<int> > ret;
        
        if(NULL == root)
            return ret;
        
        queue<TreeNode *> vi;
        
        vi.push(root);
        
        while(!vi.empty()){
            vector<int> v;
            for(int i = 0, n = vi.size(); i < n ; i++ ){
                TreeNode *tmp = vi.front();
                vi.pop();
                if(tmp->left != NULL)
                    vi.push(tmp->left);
                if(tmp->right != NULL)
                    vi.push(tmp->right);
                v.push_back(tmp->val);
            }
            ret.push_back(v);
        }
        reverse(ret.begin(),ret.end());
        return ret;
    }
};