[AcWing 851] spfa求最短路

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点击查看代码

#include<iostream>
#include<cstring>
#include<queue>

using namespace std;
const int N = 1e5 + 10;
int n, m;
int h[N], e[N], ne[N], w[N], idx;
int dist[N];
bool st[N];
void add(int a, int b, int c)
{
    e[idx] = b;
    w[idx] = c;
    ne[idx] = h[a];
    h[a] = idx++;
}
int spfa()
{
    memset(dist, 0x3f, sizeof(dist));
    dist[1] = 0;
    queue<int> q;
    q.push(1);
    st[1] = true;
    while (q.size()) {
        int t = q.front();
        q.pop();
        st[t] = false;
        for (int i = h[t]; i != -1; i = ne[i]) {
            int j = e[i];
            if (dist[j] > dist[t] + w[i]) {
                dist[j] = dist[t] + w[i];
                if (!st[j]) {
                    q.push(j);
                    st[j] = true;
                }
            }
        }
    }
    return dist[n];
}
int main()
{
    cin >> n >> m;
    memset(h, -1, sizeof(h));
    for (int i = 0; i < m; i ++) {
        int a, b, c;
        cin >> a >> b >> c;
        add(a, b, c);
    }
    int t = spfa();
    if (t == 0x3f3f3f3f)    puts("impossible");
    else    cout << dist[n] << endl;
    return 0;
}

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1. 在 dijkstra 的基础上进行优化，只有当队头节点出队时，队头的邻边才会进行更新；
2. spfa 可以用来处理负权图的情况；