• 暴力解法

#include <stdio.h>
#include <iostream>
#include <vector>
#pragma warning(disable:4996);
using namespace std;

const int maxn = 10;
int a[maxn];
bool choose[maxn];

int cal()
{
	int w = -1;
	for (int i = 0; i < 5; i++)
	{
		for (int j = i + 1; j < 5; j++)
		{
			for (int k = j + 1; k < 5; k++)
			{
				if ((a[i] + a[j] + a[k]) % 10 == 0)
				{
					w = 0;
					for (int m = 0; m < 5; m++)
					{
						if (m != i && m != j && m != k)
						{
							w = (w + a[m]) % 10;
						}
					}
					return w;
				}
			}
		}
	}
	return w;
}
int main()
{
	int m;
	scanf("%d", &m);
	while (m--)
	{
		for (int i = 0; i < 5; i++)
		{
			scanf("%d", &a[i]);
		}
		int w = cal();
		printf("%d\n", w);
	}
}

• 回溯法

#include <stdio.h>
#include <iostream>
#include <vector>
#include <algorithm>
#pragma warning(disable:4996);
using namespace std;
const int maxn = 10;
int a[maxn];
bool visit[maxn];

int calSum()
{
	int ans = 0;
	for (int i = 0; i < 5; i++)
	{
		ans += a[i];
	}
	return ans;
}

int cal(int num,int sum)
{
	if (num > 3)
		return -1;
	if (num == 3)
	{
		if (sum % 10 == 0)
		{
			int w = (calSum() - sum)%10;
			return w;
		}
		return -1;
	}
	for (int i = 0; i < 5; i++)
	{
		if (visit[i] == false)
		{
			visit[i] = true;
			int f = cal(num + 1, sum + a[i]);
			if (f != -1)
				return f;
			visit[i] = false;
		}
	}
	return -1;
}



int main()
{
	int m;
	scanf("%d", &m);
	while (m--)
	{
		fill(a, a + maxn, 0);
		fill(visit, visit + maxn, false);
		for (int i = 0; i < 5; i++)
		{
			scanf("%d", &a[i]);
		}
		
		int w = cal(0,0);
		printf("%d\n", w);
	}
}