UVA 208 Firetruck（消防车）（并查集+回溯）

题意：输入着火点n，求结点1到结点n的所有路径，按字典序输出，要求结点不能重复经过。

分析：用并查集事先判断结点1是否可以到达结点k，否则会超时。dfs即可。

#pragma comment(linker, "/STACK:102400000, 102400000")
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<cctype>
#include<cmath>
#include<iostream>
#include<sstream>
#include<iterator>
#include<algorithm>
#include<string>
#include<vector>
#include<set>
#include<map>
#include<stack>
#include<deque>
#include<queue>
#include<list>
#define Min(a, b) ((a < b) ? a : b)
#define Max(a, b) ((a < b) ? b : a)
typedef long long ll;
typedef unsigned long long llu;
const int INT_INF = 0x3f3f3f3f;
const int INT_M_INF = 0x7f7f7f7f;
const ll LL_INF = 0x3f3f3f3f3f3f3f3f;
const ll LL_M_INF = 0x7f7f7f7f7f7f7f7f;
const int dr[] = {0, 0, -1, 1, -1, -1, 1, 1};
const int dc[] = {-1, 1, 0, 0, -1, 1, -1, 1};
const int MOD = 1e9 + 7;
const double pi = acos(-1.0);
const double eps = 1e-8;
const int MAXN = 30 + 10;
const int MAXT = 10000 + 10;
using namespace std;
int fa[MAXN];
int pic[MAXN][MAXN];
int vis[MAXN];
int ans[MAXN];
int cnt;
int n;
int Find(int v){
    return fa[v] = (fa[v] == v) ? v : Find(fa[v]);
}
void dfs(int cur){
    if(ans[cur] == n){
        ++cnt;
        for(int i = 1; i <= cur; ++i){
            if(i != 1) printf(" ");
            printf("%d", ans[i]);
        }
        printf("\n");
    }
    else{
        for(int i = 1; i <= 21; ++i){
            if(pic[ans[cur]][i] && !vis[i]){
                vis[i] = 1;
                ans[cur + 1] = i;
                dfs(cur + 1);
                vis[i] = 0;
            }
        }
    }
}
int main(){
    int kase = 0;
    while(scanf("%d", &n) == 1){
        int x, y;
        memset(pic, 0, sizeof pic);
        memset(vis, 0, sizeof vis);
        memset(ans, 0, sizeof ans);
        cnt = 0;
        for(int i = 0; i < MAXN; ++i){
            fa[i] = i;
        }
        while(scanf("%d%d", &x, &y) == 2){
            if(!x && !y) break;
            pic[x][y] = 1;
            pic[y][x] = 1;
            int tx = Find(x);
            int ty = Find(y);
            if(tx < ty) fa[ty] = tx;
            else if(tx > ty) fa[tx] = ty;
        }
        printf("CASE %d:\n", ++kase);
        if(Find(n) != 1){
            printf("There are 0 routes from the firestation to streetcorner %d.\n", n);
            continue;
        }
        vis[1] = 1;
        ans[1] = 1;
        dfs(1);
        printf("There are %d routes from the firestation to streetcorner %d.\n", cnt, n);
    }
    return 0;
}