LeetCode 18. 四数之和

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        vector<vector<int>> ans;
        long long tmp=target;
        sort(nums.begin(),nums.end());
        for(int a=0;a<nums.size();a++)
        {
            if(a&&nums[a]==nums[a-1])
                continue;
            for(int b=a+1;b<nums.size();b++)
            {
                if(b!=a+1&&nums[b]==nums[b-1])
                    continue;
                for(int c=b+1,d=nums.size()-1;c<d;c++)//双指针
                {
                    if(c!=b+1&&nums[c]==nums[c-1])
                        continue;
                    while(d!=c+1&&(long long)nums[a]+nums[b]+nums[c]+nums[d-1]>=tmp)
                        d--;
                    if((long long)nums[a]+nums[b]+nums[c]+nums[d]==tmp)
                        ans.push_back({nums[a],nums[b],nums[c],nums[d]});
                }
            }
        }
        return ans;
    }
};