C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAXSIZE 100010
#define INF 0x3f3f3f3f
int n, m, dist[MAXSIZE], cnt[MAXSIZE];
int head, tail, queue[MAXSIZE], inQueue[MAXSIZE];
struct Edge {
    int weight_, to_;
    struct Edge *next_;
} *edges[MAXSIZE];

void init() {
    scanf("%d%d", &n, &m);
    int x, y, z;
    for (int i = 0; i < m; ++i) {
        scanf("%d%d%d", &x, &y, &z);
        struct Edge *s = (struct Edge*) calloc(1, sizeof(struct Edge));
        s->to_ = y;
        s->weight_ = z;
        s->next_ = edges[x];
        edges[x] = s;
    }
    memset(dist, 0x3f, sizeof(dist));
    head = -1, tail = -1;
}

int empty() {
    return head == tail ? 1 : 0;
}

void push(int val) {
    queue[++tail] = val;
}

void pop() {
    ++head;
}

int front() {
    return queue[head+1];
}

int SPFA(int start) {
    dist[start] = 0;
    push(start);
    inQueue[start] = 1;
    ++cnt[start];

    while (!empty()) {
        int u = front();
        pop();
        inQueue[u] = 0;

        struct Edge *p = edges[u];
        while (p != NULL) {
            int v = p->to_, w = p->weight_;
            p = p->next_;
            if (dist[u] + w < dist[v]) {
                dist[v] = dist[u] + w;
                if (!inQueue[v]) {
                    push(v);
                    inQueue[v] = 1;
                    ++cnt[v];
                    if (cnt[v] >= n) {
                        return 0;
                    }
                }
            }
        }
    }

    return 1;
}

int main() {
    init();
    if (SPFA(1) && dist[n] < INF/2) {
        printf("%d\n", dist[n]);
    } else {
        puts("impossible");
    }

    return 0;
}

C++

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

const int MAXSIZE = 100010;
const int INF = 0x3f3f3f3f;
int n, m;
vector<vector<pair<int, int> > > graph(MAXSIZE);
vector<int> dist(MAXSIZE, INF), cnt(MAXSIZE, 0);
vector<bool> inQueue(MAXSIZE, false);
queue<int> q;

void addEdge(int from, int to, int weight) {
    graph[from].emplace_back(weight, to);
}

void init() {
    int x, y, z;
    cin >> n >> m;
    while (m--) {
        cin >> x >> y >> z;
        addEdge(x, y, z);
    }
}

bool SPFA(int start) {
    dist[start] = 0;
    q.push(start);
    inQueue[start] = true;
    ++cnt[start];

    while (!q.empty()) {
        int u = q.front();
        q.pop();
        inQueue[u] = false;

        for (const auto [w, v] : graph[u]) {
            if (dist[u] + w < dist[v]) {
                dist[v] = dist[u] + w;
                if (!inQueue[v]) {
                    q.push(v);
                    inQueue[v] = true;
                    ++cnt[v];
                    if (cnt[v] >= n) {
                        return false;
                    }
                }
            }
        }
    }

    return true;
}

int main() {
    init();
    if (SPFA(1) && dist[n] < INF/2) {
        cout << dist[n] << endl;
    } else {
        cout << "impossible" << endl;
    }

    return 0;
}