18. 4Sum

1. 问题描述

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.

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]
]

Tags：Array Hash Table Two Pointers

Similar Problems： (E) Two Sum (M) 3Sum

2. 解答思路

3. 代码

#include <vector>
#include <map>
#include <algorithm>
#include <string>
using namespace std;
class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        vector<vector<int>> vResult;
        if (!Validate(nums))
        {
            return vResult;
        }
        std::map<std::string, bool> isExist;//用于存储已存在的四元组
        int vSize = nums.size();
        std::sort(nums.begin(), nums.end());
        int Fir, Sec, Trd, Fou;
        for (Fir=0; Fir<=vSize-4; Fir++)
        {
            int t1 = target - nums[Fir];
            for (Sec=Fir+1; Sec<=vSize-3; Sec++)
            {
                int t = t1 - nums[Sec];
                for (Trd=Sec+1, Fou=vSize-1; Trd<Fou;)
                {
                    int sum = nums[Trd] + nums[Fou];
                    if (sum < t)
                    {
                        Trd++;
                    }
                    else if (sum > t)
                    {
                        Fou--;
                    }
                    else
                    {
                        std::vector<int> vTemp;
                        vTemp.push_back(nums[Fir]);
                        vTemp.push_back(nums[Sec]);
                        vTemp.push_back(nums[Trd]);
                        vTemp.push_back(nums[Fou]);
                        
                        char pBuf[100];
                        sprintf(pBuf, "%d_%d_%d_%d", nums[Fir], nums[Sec], nums[Trd], nums[Fou]);
                        std::string str(pBuf);                        
                        if (0 == isExist.count(str))
                        {
                            isExist.insert(std::make_pair(str, true));
                            vResult.push_back(vTemp);
                        }
                        
                        Trd++;
                    }
                }
            }
        }
        return vResult;
    }
private:
    bool Validate(vector<int>& nums)
    {
        if (nums.size() < 4)
        {
            return false;
        }
        return true;
    }
};

4. 反思