算法练习：四数之和

题目链接：. - 力扣（LeetCode）

这里利用双指针来控制：

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        int n = nums.size();
        sort(nums.begin(),nums.end());
        vector<vector<int>> ret;
        for(int a = 0;a < n;)
        {
            long long sum = target - nums[a];
            for(int b = a+1;b < n; )
            {
                int left = b+1,right = n-1;
                long long sum1 = sum - nums[b];
                while(left < right)
                {
                    long long add = nums[left] + nums[right];
                    if(add == sum1)
                    {
                        ret.push_back({nums[a],nums[b],nums[left],nums[right]});
                        left++;
                        right--;
                        while(nums[left] == nums[left-1] && left < right) left++;
                        while(nums[right] == nums[right+1] && left < right) right--;
                    }
                    else if(add<sum1) left++;
                    else right--;
                }
                b++;
                while(b < n && nums[b] == nums[b-1]) b++;
            }
            a++;
            while(a < n && nums[a] == nums[a-1]) a++;
        }
        return ret;
    }
};