Permutations I

来源：Leetcode

原帖：http://oj.leetcode.com/problems/permutations/

题目：
Given a collection of numbers, return all possible permutations.
For example,
[1,2,3] have the following permutations:
[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].

代码：

 class Solution {  
 public:  
   vector<vector<int>> permute(vector<int> &num) {  
     if (num.empty()) {}  
     vector<vector<int> > result;  
     vector<bool> avail(num.size(), true);  
     vector<int> onepum;  
     permuteHelper(num, avail, onepum, result);  
     return result;  
   }  
   
   void permuteHelper(const vector<int> &num, vector<bool> &avail, vector<int> &onepum, vector<vector<int>> &result) {  
     if (onepum.size() == num.size()) {  
       result.push_back(onepum);  
       return;  
     }      
     for (int i = 0; i < num.size(); ++i) {  
       if (avail[i]) {  
         avail[i] = false;  
         onepum.push_back(num[i]);  
         permuteHelper(num, avail, onepum, result);  
         onepum.pop_back();  
         avail[i] = true;  
       }  
     }  
   }  
 };