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;    }};