【一天一道LeetCode】#18. 4Sum

一天一道LeetCode

（一）题目

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: Elements in a quadruplet (a,b,c,d) must be in non-descending
order. (ie, a ≤ b ≤ c ≤ d) 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)

（二）解题

勉强不超时的代码。(注：和3sum比较类似)

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