单源最短路板子，用spfa即可（题目数据小，好写）

#include <bits/stdc++.h>
#define sz(a, b) ((a + b - 1) / b)
#define all(_) _.begin(), _.end()
using namespace std;
using i64 = int64_t;
using u64 = uint64_t;
using u32 = uint32_t;
const int N = 2510, M = 6210;
struct edge { int to; int w; int nxt; } e[M << 1];
int n, m, st, ed;
int h[N], dist[N], idx;
bitset<N> vis;

void add(int a, int b, int c) {
  e[idx] = {b, c, h[a]};
  h[a] = idx++;
}

int spfa() {
  memset(dist, 0x3f, sizeof dist);
  queue<int> q;
  dist[st] = 0;
  q.push(st);
  vis[st] = true;
  while(q.size()) {
    int t = q.front();
    q.pop();
    vis[t] = false;
    for(int i = h[t]; ~i; i = e[i].nxt) {
      int j = e[i].to, w = e[i].w;
      if(dist[j] > dist[t] + w) {
        dist[j] = dist[t] + w;
        if(!vis[j]) {
          q.push(j);
          vis[j] = true;
        }
      }
    }
  }
  return dist[ed];
}

int main() {
  cin.tie(nullptr) -> ios::sync_with_stdio(false);
  memset(h, -1, sizeof h);
  cin >> n >> m >> st >> ed;
  for(; m --; ) {
    int a, b, c;
    cin >> a >> b >> c;
    add(a, b, c);
    add(b, a, c);
  }

  int ans = spfa();
  cout << ans << "\n";
}