18. 4Sum

18. 4Sum

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        vector<vector<int>> res;
        if(nums.size()<4){
            return res;
        }
        vector<int> vec;
        sort(nums.begin(),nums.end());
        for(int i=0;i<nums.size()-3;i++){
            if(i>0 && nums[i] == nums[i-1]){
                continue;
            }
            for(int j=i+1;j<nums.size()-2;j++){
                if(j>i+1 && nums[j] == nums[j-1]){
                    continue;
                }
                int m=j+1,n=nums.size()-1;
                int sum = target-nums[i]-nums[j];
                while(m<n){
                    if(nums[m]+nums[n]<sum){
                        m++;
                    }
                    else if(nums[m]+nums[n]>sum){
                        n--;
                    }
                    else{
                        vec.clear();
                        vec.push_back(nums[i]);
                        vec.push_back(nums[j]);
                        vec.push_back(nums[m]);
                        vec.push_back(nums[n]);
                        res.push_back(vec);
                        m++;
                        n--;
                        while(m<n && nums[m] == nums[m-1]) m++;
                        while(m<n && nums[n] == nums[n+1]) n--;
                    }
                }
                
            }
        }
        return res;
    }
};