POJ 2965 The Pilots Brothers' refrigerator（枚举+DFS）

The Pilots Brothers' refrigerator

Time Limit: 1000MS		Memory Limit: 65536K
Total Submissions: 11462		Accepted: 4200		Special Judge

Description

The game “The Pilots Brothers: following the stripy elephant” has a quest where a player needs to open a refrigerator.

There are 16 handles on the refrigerator door. Every handle can be in one of two states: open or closed. The refrigerator is open only when all handles are open. The handles are represented as a matrix 4х4. You can change the state of a handle in any location [i, j] (1 ≤ i, j ≤ 4). However, this also changes states of all handles in row i and all handles in column j.

The task is to determine the minimum number of handle switching necessary to open the refrigerator.

Input

The input contains four lines. Each of the four lines contains four characters describing the initial state of appropriate handles. A symbol “+” means that the handle is in closed state, whereas the symbol “−” means “open”. At least one of the handles is initially closed.

Output

The first line of the input contains N – the minimum number of switching. The rest N lines describe switching sequence. Each of the lines contains a row number and a column number of the matrix separated by one or more spaces. If there are several solutions, you may give any one of them.

Sample Input

-+--
----
----
-+--

Sample Output

6
1 1
1 3
1 4
4 1
4 3
4 4

Source

Northeastern Europe 2004, Western Subregion

解题报告：这道题目和1753思路一样，只是要输出所执行的步骤而已，但是很变态的是在DFS函数中不能判断flag否则超时，详见代码

代码如下：

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;
const int N = 10;
struct node
{
    int p, q;
}a[18];
int map[N][N], ans, flag;
void Change(int x, int y) //翻转函数
{
    int i;
    map[x][y] = !map[x][y];
    for (i = 1; i <= 4; ++i)
    {
        map[x][i] = !map[x][i];
        map[i][y] = !map[i][y];
    }
}
int Judge() //判断函数
{
    int i, j;
    for (i = 1; i <= 4; ++i)
    {
        for (j = 1; j <= 4; ++j)
        {
            if (map[i][j] == 0)
            {
                return 0;
            }
        }
    }
    return 1;
}
void DFS(int x, int y, int step)
{
    if (step == ans)
    {
        flag = Judge();//不能加if(flag),否则超时
        return ;
    }
    if (flag || x >= 5)
    {
        return ;
    }
    Change(x, y);
    if (y < 4)
    {
        DFS(x, y + 1, step + 1);
        a[step].p = x;
        a[step].q = y;
    }
    else
    {
        DFS(x + 1, 1, step + 1);
        a[step].p = x;
        a[step].q = y;
    }
    Change(x, y);
    if (y < 4)
    {
        DFS(x, y + 1, step);
    }
    else
    {
        DFS(x + 1, 1, step);
    }
}
int main()
{
    int i, j;
    char str;
    for (i = 1; i <= 4; ++i)
    {
        for (j = 1; j <= 4; ++j)
        {
            scanf("%c", &str);
            if (str == '-')
            {
                map[i][j] = 1;
            }
            else
            {
                map[i][j] = 0;
            }
        }
        getchar();//度掉回车
    }
    flag = 0;
    for (i = 0; i <= 16; ++i)
    {
        ans = i;
        DFS(1, 1, 0);
        if (flag)
        {
            break;
        }
    }
    if (flag)
    {
        printf("%d\n", ans);
        for (i = 0; i < ans; ++i)
        {
            printf("%d %d\n", a[i].p, a[i].q);
        }
    }
    return 0;
}