POJ2965【+，-开冰箱】简单DFS--有一步看不懂！！

The Pilots Brothers' refrigerator

Time Limit: 1000MS		Memory Limit: 65536K
Total Submissions: 21345		Accepted: 8243		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

Source

Northeastern Europe 2004, Western Subregion

题意：每次翻一点，把它横竖行（包括自己）都改变，直到全为 - ；输出步骤数和具体步骤

#include <stdio.h>
#include <math.h>
int map1[6][6],temp,l;
int way[17][2]={1};

int judge()
{
    int x=0,i,j;
    for (i=1; i<=4; i++)
    {
        for (j=1; j<=4; j++)
            x+=map1[i][j];
    }
    if(x==16)
        return 1;
    else
        return 0;
}

void convert(int x,int y)
{
    int i;
    map1[x][y]=!map1[x][y];
    for(i=1;i<=4;i++)
    {
        map1[x][i]=!map1[x][i];
        map1[i][y]=!map1[i][y];
    }
}

void dfs (int x,int y,int t,int sum,int sl)
{
    int i,j;
    if(t==sum)
    {
        temp=judge();
        l=sl;
        return ;
    }
    for(i=x;i<=4;i++)
    {
        if(i==x)
            j=1+y;        //难以理解
        else
            j=1;
        for(j;j<=4;j++)
        {
            way[sl][0]=i;
            way[sl][1]=j;
            convert(i,j);
            dfs(i,j,t+1,sum,sl+1);
            if(temp==1)return ;
            convert(i,j);
        }
    }
}
/*void dfs(int ii,int jj,int now,int total,int ll)
{
    int i,j;
    if(now==total)
    {
        temp=judge();
        l=ll;
        return;
    }
    for(i=ii;i<=4;i++)
    {
        if(i==ii) j=jj+1;
        else j=1;
        for(;j<=4;j++)
        {
            way[ll][0]=i;
            way[ll][1]=j;
            convert(i,j);
            dfs(i,j,now+1,total,ll+1);
            if(temp==1) return;
            convert(i,j);
        }
    }
}*/
int main ()
{
    int i,j;
    char s;
    for (i=1; i<=4; i++)
    {
        for (j=1; j<=4; j++)
        {
            scanf("%c",&s);

            if (s=='+')
                map1[i][j]=0;
            else
                map1[i][j]=1;
        }getchar();
    }
    for(i=1;i<=16;i++)
    {
        temp=0;
        dfs(1,0,0,i,1);
        if(temp==1)break;
    }
    printf("%d
",l-1);
    for(i=1;i<=l-1;i++)
        printf("%d %d
",way[i][0],way[i][1]);
    return 0;
}