LeetCode 题解（17）: 4Sum

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)

题解：

先对数组排序，利用STL set 解决了重复 quadruplet 问题, 再把4sum分解为 1个元素+3Sum，再进一步分解为 1个元素 + 另1个元素 + 2Sum，再利用2Sum求解。336ms过。

class Solution {
public:
    vector<vector<int>> fourSum(vector<int> &num, int target) {
        vector<vector<int>> results;
        if(num.size() < 4)
            return results;
        sort(num.begin(), num.end());
        set<vector<int>> result;
        for(int i = 0; i < num.size() - 3; i++)
        {
            int sum = target - num[i];
            for(int j = i + 1; j < num.size() - 2; j++)
            {
                sum -= num[j];
                //2-sum here
                int start = j + 1;
                int end = num.size() - 1;
                while(start < end)
                {
                    if(num[start] + num[end] == sum)
                    {
                        vector<int> temp;
                        temp.push_back(num[i]);
                        temp.push_back(num[j]);
                        temp.push_back(num[start]);
                        temp.push_back(num[end]);
                        result.insert(temp);
                        start++;
                        end--;
                    }
                    else if(num[start] + num[end] < sum)
                        start++;
                    else
                        end--;
                }
                sum += num[j];
            }
        }
        
        for(auto iter: result)
        {
            results.push_back(iter);
        }
        return results;
    }
};