回溯算法----全排列

class Solution {
public:
    vector<vector<int>> vec;
    void backTrace(vector<int> nums, int pos) {
        if (pos == nums.size()) {
            vec.push_back(nums);
            return;
        }
        
        for (int i = pos; i <　nums.size(); ++i) {
            std::swap(nums[pos], nums[i]);
            backTrace(nums, pos+1);
            std::swap(nums[i], nums[pos]);
        }
    }
    
    vector<vector<int>> permute(vector<int>& nums) {
        backTrace(nums, 0);
        return vec;
    }
};