这个题目比较简单
先求出直径
然后遍历所有的点，（不必检查直径的端点），$a, b$ 为直径两个端点，假设任意一个点到 $a$ 的距离为 $da$，到 $b$ 的距离为 $db$，则
$res = \max(res, \ \min(da(i), db(i)))$
$ans = res + len$

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <algorithm>
#include <queue>
#include <vector>
#include <stack>
#include <map>
#include <set>
#include <sstream>
#include <iomanip>
#include <cmath>
#include <bitset>
#include <assert.h>
#include <unordered_map>

using namespace std;
typedef long long ll;

#define Cmp(a, b) memcmp(a, b, sizeof(b))
#define Cpy(a, b) memcpy(a, b, sizeof(b))
#define Set(a, v) memset(a, v, sizeof(a))
#define debug(x) cout << #x << ": " << x << endl
#define _forS(i, l, r) for(set<int>::iterator i = (l); i != (r); i++)
#define _rep(i, l, r) for(int i = (l); i <= (r); i++)
#define _for(i, l, r) for(int i = (l); i < (r); i++)
#define _forDown(i, l, r) for(int i = (l); i >= r; i--)
#define debug_(ch, i) printf(#ch"[%d]: %d\n", i, ch[i])
#define debug_m(mp, p) printf(#mp"[%d]: %d\n", p->first, p->second)
#define debugS(str) cout << "dbg: " << str << endl;
#define debugArr(arr, x, y) _for(i, 0, x) { _for(j, 0, y) printf("%c", arr[i][j]); printf("\n"); }
#define _forPlus(i, l, d, r) for(int i = (l); i + d < (r); i++)
#define lowbit(i) (i & (-i))
#define MPR(a, b) make_pair(a, b)

pair<int, int> crack(int n) {
    int st = sqrt(n);
    int fac = n / st;
    while (n % st) {
        st += 1;
        fac = n / st;
    }

    return make_pair(st, fac);
}

inline ll qpow(ll a, int n) {
    ll ans = 1;
    for(; n; n >>= 1) {
        if(n & 1) ans *= 1ll * a;
        a *= a;
    }
    return ans;
}

template <class T>
inline bool chmax(T& a, T b) {
    if(a < b) {
        a = b;
        return true;
    }
    return false;
}

ll gcd(ll a, ll b) {
    return b == 0 ? a : gcd(b, a % b);
}

ll ksc(ll a, ll b, ll mod) {
    ll ans = 0;
    for(; b; b >>= 1) {
        if (b & 1) ans = (ans + a) % mod;
        a = (a * 2) % mod;
    }
    return ans;
}

ll ksm(ll a, ll b, ll mod) {
    ll ans = 1 % mod;
    a %= mod;

    for(; b; b >>= 1) {
        if (b & 1) ans = ksc(ans, a, mod);
        a = ksc(a, a, mod);
    }

    return ans;
}

template <class T>
inline bool chmin(T& a, T b) {
    if(a > b) {
        a = b;
        return true;
    }
    return false;
}

template<class T>
bool lexSmaller(vector<T> a, vector<T> b) {
    int n = a.size(), m = b.size();
    int i;
    for(i = 0; i < n && i < m; i++) {
        if (a[i] < b[i]) return true;
        else if (b[i] < a[i]) return false;
    }
    return (i == n && i < m);
}

// ============================================================== //

const int maxn = 200000 + 10;
const int maxm = 400000 + 10;
const ll inf = 0x3f3f3f3f3f3f3f3f;
int head[maxn], ver[maxm], ne[maxm], tot = 1, n, m;
ll edges[maxm];

void add(int x, int y, ll z) {
    ver[++tot] = y; edges[tot] = z; ne[tot] = head[x]; head[x] = tot;
}

int vis[maxn];
ll d[maxn], da[maxn], db[maxn];

void bfs(int u, int &pos) {
    memset(vis, 0, sizeof vis);
    memset(d, 0, sizeof d);
    vis[u] = 1;
    queue<int> q; q.push(u);

    while (q.size()) {
        int x = q.front(); q.pop();
        for (int i = head[x]; i; i = ne[i]) {
            int y = ver[i];
            if (vis[y]) continue;
            vis[y] = 1;
            d[y] = d[x] + edges[i];
            if (d[y] > d[pos]) pos = y;
            q.push(y);
        }
    }
}

void diameter() {
    int a = 0;
    bfs(1, a);
    int b = 0;
    bfs(a, b);

    // [a, b] diameter
    ll len = d[b];

    // a start
    memcpy(da, d, sizeof d);
    // b start
    int t = 0;
    bfs(b, t);
    memcpy(db, d, sizeof d);

    ll res = 0;
    for (int i = 1; i <= n; i++) {
        if (i != a && i != b) {
            res = max(res, min(da[i], db[i]));
        }
    }

    ll ans = res + len;
    printf("%lld\n", ans);
}

int main() {
    //freopen("input.txt", "r", stdin);
    scanf("%d%d", &n, &m);
    for (int i = 0; i < m; i++) {
        int x, y;
        ll z;
        scanf("%d%d%lld", &x, &y, &z);
        add(x, y, z); add(y, x, z);
    }

    // bfs and diameter
    diameter();
}