给定一个数字列表，返回其所有可能的排列。

class Solution {
public:
    /*
     * @param nums: A list of integers.
     * @return: A list of permutations.
     */
    vector<vector<int>> permute(vector<int> &nums) {
        // write your code here
        vector<vector<int>> result;
        sort(nums.begin(),nums.end());
        permuteRec( 0, nums, result);
        return result;
    }
    
    void permuteRec( int start, vector<int> nums, vector<vector<int>> &result ){
        if( start >= nums.size()){
            result.push_back(nums);
            return;
        }
        for( int i = start; i < nums.size(); i++ ){
            if( i > start && nums[i] == nums[start]) continue;
            swap( nums[start], nums[i] );
            permuteRec( start+1, nums, result);
        }
    }
};