ZOJ 1002 回溯

題目：http://acm.zju.edu.cn/onlinejudge/showProblem.do?problemId=2

#include<cstdio>#include<cstring>#include<algorithm>using namespace std;int a[5][5], w[5][5];int n, ans;bool check(int r, int c){    int i;    if(w[r][c] == 1) return false;    for(i = r - 1; i >= 0; i--)    {        if(a[i][c] == 1) return false;        if(w[i][c] == 1) break;    }    for(i = r + 1; i < n; i++)    {        if(a[i][c] == 1) return false;        if(w[i][c] == 1) break;    }    for(i = c - 1; i >= 0; i--)    {        if(a[r][i] == 1) return false;        if(w[r][i] == 1) break;    }    for(i = c + 1; i < n; i++)    {        if(a[r][i] == 1) return false;        if(w[r][i] == 1) break;    }    return true;}void dfs(int index, int cnt){    if(index == n * n)    {        ans = max(ans, cnt);        return;    }    if(ans > n * n - index + cnt) return;    int r = index / n;    int c = index % n;    if(check(r, c))    {        a[r][c] = 1;        dfs(index + 1, cnt + 1);        a[r][c] = 0;        dfs(index + 1, cnt);        return;    }    dfs(index + 1, cnt);}int main(){    while(scanf("%d",&n))    {        if(n == 0) break;        memset(a, 0, sizeof(a));        memset(w, 0, sizeof(w));        char str[10];        for(int i = 0; i < n; i++)        {            scanf("%s",str);            for(int j = 0; j < n; j++)                if(str[j] == 'X')                    w[i][j] = 1;        }        ans = 0;        dfs(0, 0);        printf("%d\n",ans);    }    return 0;}