[HDOJ2544]最短路（SPFA）

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=2544

最短路，学了写SPFA，判断负环很方便。

#include <cstdio>
#include <queue>
#include <vector>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
typedef pair<int, int> pii;

const int maxn = 110;
const int maxm = 10100;
const int inf = 0x7f7f7f;
const int nl = 100;
vector<pii> G[maxn];
int vcnt[maxn];
int d[maxn];
queue<int> q;
int n, m;

bool spfa(int s) {
    for(int i = 1; i <= n; i++) d[i] = inf;
    memset(vcnt, 0, sizeof(vcnt));
    d[s] = 0; q.push(s);
    while(!q.empty()) {
        int u = q.front(); q.pop();
        for(int i = 0; i < G[u].size(); i++) {
            int v = G[u][i].first;
            int w = G[u][i].second;
            if(d[v] > d[u] + w) {
                d[v] = d[u] + w;
                vcnt[v]++;
                q.push(v);
                if(vcnt[v] > nl) return 0;
            }
        }
    }
    return 1;
}

int main() {
    // freopen("in", "r", stdin);
    int u, v, w;
    while(~scanf("%d%d",&n,&m) && n + m) {
        for(int i = 1; i <= n; i++) G[i].clear();
        for(int i = 0; i < m; i++) {
            scanf("%d%d%d",&u,&v,&w);
            G[u].push_back(pii(v, w));
            G[v].push_back(pii(u, w));
        }
        spfa(1);
        printf("%d
", d[n]);
    }
    return 0;
}