leetcode 18.四数之和

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        vector<vector<int>> res;
        sort(nums.begin(), nums.end());
        for(int k=0;k<nums.size();k++){
            if(nums[k]>=0&&nums[k]>target){
                break;
            }
            if(k>0&&nums[k-1]==nums[k]){
                continue;
            }

            for(int i=k+1;i<nums.size();i++){
                if(((nums[i]+nums[k])>=0)&&(nums[i]+nums[k])>target){
                    break;
                }
                if(i>k+1&&nums[i]==nums[i-1]) continue;

                int left=i+1;
                int right=nums.size()-1;
                while(left<right){
                    long long int sum=(long)nums[k] + nums[i] + nums[left] + nums[right];
                    if(sum == target){
                        res.push_back(vector<int>{nums[k],nums[i],nums[left],nums[right]});
                        while(right>left&&nums[right]==nums[right-1]){right--;}
                        while(right>left&&nums[left]==nums[left+1]){left++;}

                        right--;
                        left++;
                    }else if(sum > target){
                        right--;
                    }else{
                        left++;
                    }
                }
            }
            
        }return res;
    }
};

和三数之和类似，需注意剪枝顺序，同时需注意vector的插入方法。同时continue不能被i++/k++替代。