排列问题

class Solution {
public:
    vector<vector<int>> permute(vector<int>& nums) {
        vector<vector<int>> result;
        help(nums, 0, result);
        return result;
    }
    
    // permute num[begin..end]
    // invariant: num[0..begin-1] have been fixed/permuted
    void help(vector<int>& nums, int begin, vector<vector<int>>& result){
        if(begin >= nums.size()){
            // one permutation instance
            result.push_back(nums);
            return;
        }
        for(int i = begin; i < nums.size(); ++i){
            swap(nums[i], nums[begin]);
            help(nums, begin + 1, result);
            // reset
            swap(nums[i], nums[begin]);
        }
    }
};

class Solution {
public:
    vector<vector<int>> permuteUnique(vector<int>& nums) {
        vector<vector<int>> result;
        sort(nums.begin(), nums.end());
        help(nums, 0, result);
        return result;
    }
    
    void help(vector<int> nums, int begin, vector<vector<int>>& result){
        if(begin == nums.size() - 1){
            result.push_back(nums);
            return;
        }
        for(int i = begin; i < nums.size(); ++i){
            if(i != begin && nums[i] == nums[begin]) continue;
            else{
                swap(nums[begin], nums[i]);
                help(nums, begin + 1, result);
            }
        }
    }
};