[leetcode]4Sum

题目： https://oj.leetcode.com/problems/4sum/

代码：

对于这类问题首先需要将数组进行整体排序。然后比较某两个数是否存在和target-这两个数和 相等的两个数。

值得注意的是，要考虑到重复的情况，所以需要用set之类作区分。

class Solution {
public:
    vector<vector<int> > fourSum(vector<int> &num, int target) {
		vector< vector<int> > res;

		sort(num.begin(), num.end());
		int size = num.size();
		set<int>s;
		for (int i = 0; i < size; i++)
		{
			if (s.find(num[i]) == s.end())
			{
				s.insert(num[i]);
				for (int j = i + 1; j < size; j++)
				{
					while (j != i + 1 && num[j] == num[j - 1] && j<size) j++;
					get_triplus(i, j, num, res, j + 1, size - 1, target);
				}
			}
		}
		return res;
	}

	void get_triplus(int indexone, int indextwo, vector<int>&num, vector< vector<int> > &res, int begin, int end, int target)
	{
		while (begin < end)
		{
			if (num[indexone] + num[indextwo] + num[begin] + num[end] == target)
			{
				putRes(num[indexone], num[indextwo], num[begin], num[end], res);
				while (begin < end && num[begin] == num[begin + 1]) begin++;
				begin++;
				while (begin < end && num[end] == num[end - 1]) end--;
				end--;
			}
			else if (num[indexone] + num[indextwo] + num[begin] + num[end] > target) end--;
			else begin++;
		}
	}

	void putRes(int a, int b, int c, int d, vector< vector<int> >& res)
	{
		vector<int>tmp;
		tmp.push_back(a);
		tmp.push_back(b);
		tmp.push_back(c);
		tmp.push_back(d);
		res.push_back(tmp);
	}
};