UVA208回溯法水题

枚举所有可能的路径

要注意的是：

1》并查集判联通

2》如果一个点的下一个点就是goal，那么当直接判断出来后，不能够直接返回，应该去continue

下面是AC代码

#include<cstdio>
#include<cstring>
using namespace std;

const int maxn = 30;
int G[maxn][maxn];

int goal;
int cnt = 0;
int pa[maxn];
int find(int x)
{
    if(pa[x] != x)
    {
        pa[x] = find(pa[x]);
    }
    return pa[x];
}

bool read_G()
{
    //读图
    memset(pa,0,sizeof(pa));
    memset(G,0,sizeof(G));
    for(int i = 1;i <= 21;i++)
    {
        pa[i] = i;//初始化并查集
    }
    if(scanf("%d",&goal) != 1)
        return false;
    int x1,x2;
    while(scanf("%d%d",&x1,&x2) == 2 && x1)
    {
        G[x1][x2] = G[x2][x1] = 1;
        int s1 = find(x1),s2 = find(x2);
        if(s1 != s2)//直接联通
        {
            pa[s1] = pa[s2];
        }
    }
    return true;
}
int ans[maxn];
int vis[maxn];


void print_ans(int pos)
{
    cnt++;
    for(int i = 0;i <= pos;i++)
    {
        if(i)
            printf(" ");
        printf("%d",ans[i]);
    }
    printf("\n");
}
void dfs(int cur)
{
    for(int i = 1;i <= 21;i++)
    {
        if(G[ans[cur - 1]][i] == 1 && !vis[i])
        {
            vis[i] = 1;
            ans[cur] = i;
            if(i == goal)
            {
                print_ans(cur);
                vis[i] = 0;
                continue;//这个地方刚才出错了
            }
            dfs(cur + 1);
            vis[i] = 0;
        }
    }
}

int main()
{
    //freopen("input.txt","r",stdin);
    //freopen("out.txt","w",stdout);
    int kase = 0;
    while(read_G())
    {
        cnt = 0;
        printf("CASE %d:\n",++kase);
        //注意两个点联通和并查集判断联通块个数的区别
        if(find(1) != find(goal))
        {
            printf("There are 0 routes from the firestation to streetcorner %d.\n",goal);
            continue;
        }
        else if(goal == 1)
        {
            printf("1\n");
            printf("There are 1 routes from the firestation to streetcorner %d.\n",goal);
            continue;
        }
        memset(vis,0,sizeof(vis));
        ans[0] = 1;
        vis[1] = 1;
        dfs(1);
        printf("There are %d routes from the firestation to streetcorner %d.\n",cnt,goal);
    }
    return 0;
}