18. 4Sum(C++)

Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.

Note: The solution set must not contain duplicate quadruplets.

For example, given array S = [1, 0, -1, 0, -2, 2], and target = 0.

A solution set is:
[
  [-1,  0, 0, 1],
  [-2, -1, 1, 2],
  [-2,  0, 0, 2]
]

Solution :

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        vector<vector<int>> res;
        if(nums.empty()) return res;
        std::sort(nums.begin(),nums.end());
        int len=nums.size();
        for(int i1=0;i1<len;i1++){
            for(int i2=i1+1;i2<len;i2++){
                int i3=i2+1;
                int i4=len-1;
                while(i3<i4){
                    int sum=nums[i1]+nums[i2]+nums[i3]+nums[i4];
                    if(sum>target){
                        i4--;
                    }else if(sum<target){
                        i3++;
                    }else{
                        vector<int> tmp(4,0);
                        tmp[0]=nums[i1];
                        tmp[1]=nums[i2];
                        tmp[2]=nums[i3];
                        tmp[3]=nums[i4];
                        res.push_back(tmp);
                        while(i3<i4&&nums[i3]==tmp[2]) i3++;
                        while(i3<i4&&nums[i4]==tmp[3]) i4--;
                    }
                }
                while(i2+1<len&&nums[i2]==nums[i2+1]) i2++;
            }
            while(i1+1<len&&nums[i1]==nums[i1+1]) i1++;
        }
        return res;
    }
};