最短路：spfa算法

题目描述

参考代码

输入示例

3 3
1 2 5
2 3 -3
1 3 4

输出示例

2

#include <iostream>
#include <cstring>
#include <algorithm>
#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; ne[idx] = h[a]; w[idx] = c; h[a] = idx; idx++;
}int spfa()
{memset(dist, 0x3f, sizeof dist);dist[1] = 0;queue<int> q;q.push(1);st[1] = true;while (q.size()){auto 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;}}}}if (dist[n] == 0x3f3f3f3f) return -1;return dist[n];
}int main()
{scanf("%d%d", &n, &m);memset(h, -1, sizeof h);while (m -- ){int x, y, z;scanf("%d%d%d", &x, &y, &z);add(x, y, z);}int t = spfa();if (t == -1) printf("impossible\n");else printf("%d\n", t);return 0;
}