[luoguP2761] 软件补丁问题（状压最短路）

传送门

n <= 20 很小

所以可以状态压缩

然后因为可能存在环，所以不能DP

那么就用spfa找最短路

被位运算坑了，不清楚优先级一定要加括号

——代码

#include <queue>
#include <cstdio>
#include <cstring>
#include <iostream>
#define M 301
#define N 4000001

int n, m;
int b1[M], b2[M], f1[M], f2[M], a[M], dis[N];
bool vis[N];

inline void spfa()
{
    int i, v, u;
    std::queue <int> q;
    memset(dis, 0x33, sizeof(dis));
    q.push((1 << n) - 1);
    dis[(1 << n) - 1] = 0;
    while(!q.empty())
    {
        u = q.front(), q.pop();
        vis[u] = 0;
        for(i = 1; i <= m; i++)
            if((u | b1[i]) == u && !(u & b2[i]))
            {
                v = u ^ (u & f1[i]) | f2[i];
                if(dis[v] > dis[u] + a[i])
                {
                    dis[v] = dis[u] + a[i];
                    if(!vis[v])
                    {
                        q.push(v);
                        vis[v] = 1;
                    }
                }
            }
    }
}

int main()
{
    int i, j;
    char s1[M], s2[M];
    scanf("%d %d", &n, &m);
    for(i = 1; i <= m; i++)
    {
        scanf("%d %s %s", &a[i], s1, s2);
        for(j = 0; j < n; j++)
        {
            if(s1[j] == '+') b1[i] |= 1 << j;
            if(s1[j] == '-') b2[i] |= 1 << j;
            if(s2[j] == '-') f1[i] |= 1 << j;
            if(s2[j] == '+') f2[i] |= 1 << j;
        }
    }
    spfa();
    dis[0] == dis[1 << n] ? puts("0") : printf("%d
", dis[0]);
    return 0;
}