[BZOJ1579] [Usaco2009 Feb]Revamping Trails 道路升级（分层图最短路 + 堆优化dijk）

#include <queue>
#include <cstdio>
#include <cstring>
#include <iostream>
#define N 50001

using namespace std;

struct node
{
    int a, b, c;
    node(int a, int b, int c) : a(a), b(b), c(c) {}
    bool operator > (const node& gg) const
    {    
        return a > gg.a;
    }
};

int n, m, k, cnt;
int head[N], to[N << 1], next[N << 1], val[N << 1], dis[N][21];
bool vis[N][21];
priority_queue <node, vector <node>, greater <node> > q;

inline int read()
{
    int x = 0, f = 1;
    char ch = getchar();
    for(; !isdigit(ch); ch = getchar()) if(ch == ‘-‘) f = -1;
    for(; isdigit(ch); ch = getchar()) x = (x << 1) + (x << 3) + ch - ‘0‘;
    return x * f;
}

inline void add(int x, int y, int z)
{
    to[cnt] = y;
    val[cnt] = z;
    next[cnt] = head[x];
    head[x] = cnt++;
}

inline void dijkstra()
{
    int i, u, v, x;
    memset(dis, 127, sizeof(dis));
    q.push(node(0, 1, 0));
    dis[1][0] = 0;
    while(!q.empty())
    {
        u = q.top().b;
        x = q.top().c;
        q.pop();
        if(vis[u][x]) continue;
        vis[u][x] = 1;
        for(i = head[u]; i ^ -1; i = next[i])
        {
            v = to[i];
            if(dis[v][x] > dis[u][x] + val[i])
            {
                dis[v][x] = dis[u][x] + val[i];
                q.push(node(dis[v][x], v, x));
            }
            if(x + 1 <= k && dis[v][x + 1] > dis[u][x])
            {
                dis[v][x + 1] = dis[u][x];
                q.push(node(dis[v][x + 1], v, x + 1));
            }
        }
    }
}

int main()
{
    int i, x, y, z;
    n = read();
    m = read();
    k = read();
    memset(head, -1, sizeof(head));
    for(i = 1; i <= m; i++)
    {
        x = read();
        y = read();
        z = read();
        add(x, y, z);
        add(y, x, z);
    }
    dijkstra();
    printf("%d\n", dis[n][k]);
    return 0;
}