[AcWing 340] 通信线路

#include<bits/stdc++.h>

using namespace std;

typedef long long LL;

const int N = 1e6 + 10;
const int M = 1e3 + 10;
const int INF = 0x3f3f3f3f;

int n, m, k;
int h[N], e[N], ne[N], w[N], idx;
int d[M];
bool st[M];

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

bool check(int x)
{
    deque<int> q;
    memset(d, 0x3f, sizeof d);
    memset(st, false, sizeof st);
    q.push_back(1);
    d[1] = 0;
    while (q.size()) {
        auto t = q.front();
        q.pop_front();
        if (st[t])
            continue;
        st[t] = true;
        for (int i = h[t]; i != -1; i = ne[i]) {
            int j = e[i];
            int dist = w[i] > x;
            if (d[j] > d[t] + dist) {
                d[j] = d[t] + dist;
                if (!dist)
                    q.push_front(j);
                else
                    q.push_back(j);
            }
        }
    }
    return d[n] <= k;
}

void solve()
{
    cin >> n >> m >> k;
    memset(h, -1, sizeof h);
    for (int i = 0; i < m; i ++) {
        int a, b, c;
        cin >> a >> b >> c;
        add(a, b, c);
        add(b, a, c);
    }
    int l = 0, r = 1e9;
    while (l < r) {
        int mid = l + r >> 1;
        if (check(mid))
            r = mid;
        else
            l = mid + 1;
    }
    if (l > 1e6)
        cout << "-1" << endl;
    else
        cout << l << endl;
}

signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    solve();

    return 0;
}