[C++知识库]AcWing851 spfa求最短路

#include<iostream>
#include<queue>
#include<cstring>
using namespace std;

const int N = 1e5 + 10;
int h[N], e[N], w[N], ne[N], idx = 0;
int dist[N];//各点到源点的距离
bool st[N];
int n, m;
void add(int a, int b, int c) {
	e[idx] = b; w[idx] = c; ne[idx] = h[a]; h[a] = idx++;
}

int spfa() {
	queue<int>q;
	memset(dist, 0x3f3f3f3f, sizeof dist);
	dist[1] = 0;
	q.push(1);
	st[1] = true;
	while (q.size()) {
		int t = q.front();
		q.pop();
		st[t] = false;//取出这个点
		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]) {
					//如果这个点不在队列里面
					q.push(j);
					st[j] = true;
					//只记录发生更新的点

				}
			}
		}
	}
	return dist[n];
}

int main() {
	scanf("%d%d", &n, &m);
	memset(h, -1, sizeof h);
	while (m--) {
		int a, b, c;
		scanf("%d%d%d", &a, &b, &c);
		add(a, b, c);
	}
	int res = spfa();
	if (res == 0x3f3f3f3f) puts("impossible");
	else printf("%d", res);

	return 0;
}