[数据结构与算法]leetcode18. 四数之和

题目：18. 四数之和

排序+双指针

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        int n = nums.size();
        vector<vector<int>> vec;
        if(n < 4) return vec;

        sort(nums.begin(), nums.end());
        for(int a=0; a<n-3; a++)
        {
            if(a > 0 && nums[a] == nums[a-1]) continue;
            if((long)nums[a] + nums[a+1] + nums[a+2] + nums[a+3] > (long)target) break;
            if(nums[a] + nums[n-3] + nums[n-2] + nums[n-1] < target) continue;
            for(int b=a+1; b<n-2; b++)
            {
                if(b > a+1 && nums[b] == nums[b-1]) continue;
                //if(nums[a] + nums[b] + nums[b+1] + nums[b+2] > target) break;
                if(nums[a] + nums[b] + nums[n-2] + nums[n-1] < target) continue;
                int c = b+1, d = n-1;
                while(c < d)
                {
                    int ans = nums[a] + nums[b] + nums[c] + nums[d];
                    if(ans == target)
                    {
                        vec.push_back({nums[a], nums[b], nums[c], nums[d]});
                        c++;
                        d--;
                        while(c < d && nums[c] == nums[c-1]) c++;
                        while(c < d && nums[d] == nums[d+1]) d--;
                    }
                    else if(ans < target) c++;
                    else d--;
                }
            }
        }

        return vec;
    }
};