能过样例但是提交全WA

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Code:

#include <iostream>
#include <algorithm>
#include <cstdio>
#include <vector>
#include <cmath>

using namespace std;
using ll = long long;
using LL = ll;
const int N = 5e3 + 10;
const int M = 2e5 + 5;
const int INF = 0x7fffffff;
inline void IOS() {
    ios::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);
}

template <typename T>
void read(T &x) {
    x = 0; char c = getchar(); int f = 0;
    for (; !isdigit(c); c = getchar())
        f |= c == '-';
    for (; isdigit(c); c = getchar())
        x = x * 10 + (c ^ '0');
    if (f) x = -x;
}
template <typename T, typename ... Args>
void read(T& a, Args&... args) {
    read(a), read(args...);
}

template <typename T>
void write(T x, char ed = '\n') {
    if (x < 0) putchar('-'), x = -x;
    static short st[30], tp;
    do st[++tp] = x % 10, x /= 10; while (x);
    while (tp) putchar(st[tp--] | '0');
    putchar(ed);
}

template <typename T, typename ... Args>
void write(T& a, Args&... args) {
    write(a), write(args...);
}

void Solve();
void merge(int a, int b);
int find(int a);

struct Edge{
    int u, v, w, nxt;
    bool operator < (const Edge b) const {
        return w < b.w;
    }
}E[M];

int head[N], cnt = 0;
void add(const Edge a)//加边，u起点，v终点，w边权
{
    E[++cnt] = Edge{a.u, a.v, a.w, head[a.u]};
    head[a.u] = cnt;
}

int n, m, ans = 0;
int u[N], fa[N];

signed main() {
    IOS();
    Solve();
    return 0;
}

void Solve() {
    read(n, m);
    for(int i = 1; i < m; ++i) {
        int u, v, w;
        read(u, v, w);
        add(Edge{u, v, w});
    }
    sort(E + 1, E + n + 1);

    for(int i = 1; i <= n; ++i) {
        fa[i] = i;
    }
    int ans = 0, tot = 0;
    for(int i = 1; i <= m; ++i) {
        int x = E[i].u, y = E[i].v;
        if(find(x) != find(y)) {
            ans += E[i].w;
            tot += 1;
            merge(x, y);
            if(tot == n - 1) break;
        }
    }
    if(ans == 0) puts("orz");
    else write(ans);
}

void merge(int a, int b) {
    fa[find(a)] = find(b);
}

int find(int a) {
    while(a != fa[a]) a = fa[a] = fa[fa[a]];
    return a;
}