18. 4Sum

枚举第一个数和最后一个数，中间两个数二分查找，也可以用set集合去重

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        int n=nums.size();
        int i,j,k,t;
        sort(nums.begin(),nums.end());
        
        vector<vector<int>> res;
        for(i=0;i<=n-4;i++)
        {
            if(i>0&&nums[i]==nums[i-1])
                continue;
            for(t=n-1;t>=i+3;t--)
            {
                if(t<n-1&&nums[t]==nums[t+1])
                    continue;
                for(j=i+1,k=t-1;j<k;)
                {
                    if(j>i+1&&nums[j]==nums[j-1])
                    {
                        j++;
                        continue;
                    }
                    if(k<t-1&&nums[k]==nums[k+1])
                    {
                        k--;
                        continue;
                    }
                    int d=nums[i]+nums[t]+nums[j]+nums[k];
                    if(d==target)
                    {
                        vector<int> vt;
                        vt.push_back(nums[i]);
                        vt.push_back(nums[j]);
                        vt.push_back(nums[k]);
                        vt.push_back(nums[t]);
                        res.push_back(vt);
                        j++;
                        k--;
                    }
                    else if(d>target)
                        k--;
                    else
                        j++;
                }
            }
        }
        return res;
    }
};