LeetCode 18. 4Sum

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
       
        sort(nums.begin(),nums.end());
        int length = nums.size();
        
        vector<vector<int>> ans;
        vector<int> res;
        for(int i=0;i<length;i++)
        {
            if(i==0||nums[i]!=nums[i-1]){
            for(int j=i+1;j<length;j++)
            {
                if(j==i+1||nums[j]!=nums[j-1])
                {
                    int start = j+1;
                    int end = length-1;
                    while(start<end)
                    {
                        res.clear();
                        if(nums[i]+nums[j]+nums[start]+nums[end]<target)
                        {
                            start++;
                            continue;
                        }
                        if(nums[i]+nums[j]+nums[start]+nums[end]>target)
                        {
                            end--;
                            continue;
                        }
                        if(nums[i]+nums[j]+nums[start]+nums[end]==target)
                        {
                            res.push_back(nums[i]);
                            res.push_back(nums[j]);
                            res.push_back(nums[start]);
                            res.push_back(nums[end]);
                            ans.push_back(res);
                            
                            while(start<end&&nums[end]==nums[end-1])
                                end--;
                            while(start<end&&nums[start]==nums[start+1])
                                start++;
                            
                            start++;
                            end--;
                        }
                    }
                }
            }
            }
        }
        return ans;
    }
};