LeetCode---(18) 4 Sum

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Problem:

Given an array S of n integers, are there elements abc, 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)
class Solution {public:    vector<vector<int>> fourSum(vector<int>& nums, int target) {        vector<vector<int>> result;        result.clear();                if(nums.size()<4)            return result;        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 index1=j+1;                int index2=nums.size()-1;                                while(index1<index2)                {                    if(index1>j+1&&nums[index1]==nums[index1-1])                    {                        index1++;                        continue;                    }                                            if(index2<nums.size()-1&&nums[index2]==nums[index2+1])                    {                        index2--;                        continue;                    }                                        int sum=nums[i]+nums[j]+nums[index1]+nums[index2];                    if(sum>target)                        index2--;                    else if(sum<target)                        index1++;                    else{                        vector<int> mid;                        mid.push_back(nums[i]);                        mid.push_back(nums[j]);                        mid.push_back(nums[index1]);                        mid.push_back(nums[index2]);                        result.push_back(mid);                        index1++;                                            }                                    }            }        }        return result;    }};

K Sum求解方法, 适用leetcode 2Sum, 3Sum, 4Sum:

方法一: 暴力,就是枚举所有的K-subset, 那么这样的复杂度就是 从N选出K个,复杂度是O(N^K)

方法二: 排序,这个算法可以考虑最简单的case, 2sum,这是个经典问题,方法就是先排序,然后利用头尾指针找到两个数使得他们的和等于target

方法三: 使用哈希表

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