从N个数中取M个和为指定值的数

 一个含有n个元素的数组，找出N个数使其和为target

思路：

     这里使用递归求解

#include <vector>
#include <iostream>
#include <algorithm>

using namespace std;

bool calNextNumber(int start, int target, int N, vector<int>&ivec, vector<int>arr)
{
	if (target == 0 && N == 0)
		return true;
	if (start + N >= arr.size() || target <= 0)
		return false;
	for (int i = start; i < arr.size(); ++i)
	{
		ivec.push_back(arr[start]);
		if (calNextNumber(i + 1, target - arr[start], N - 1, ivec, arr))
		{
			return true;
		}
		ivec.pop_back();
	}
	return false;
}

vector<vector<int>> cal(vector<int> arr, int target, int N)
{
	int length = arr.size();
	vector<vector<int>> iivec;
	vector<int> ivec;
	for (int i = 0; i < arr.size(); ++i)
	{
		if (calNextNumber(i, target, N, ivec, arr))
			iivec.push_back(ivec);
		ivec.clear();
	}
	return iivec;
}

int main()
{
	vector<int> arr = { 1, 3, 5, 9, 4, 2 };
	sort(arr.begin(), arr.end());
	vector<vector<int>> result = cal(arr, 6, 2);
	for (int i = 0; i < result.size(); ++i)
	{
		for (auto it : result[i])
			cout << it << '\t';
		cout << endl;
	}
    return 0;
}