Leetcode 18. 4Sum

18. 4Sum

• Total Accepted: 80288
• Total Submissions: 328279
• Difficulty: Medium

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.

思路：基本思想和Leetcode 16. 3Sum Closest类似，只是数的个数变为4。

代码：

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