k短路通常用 A^* 求，但是 vfk 說二分也可以。

千古神犇 vfk，撲通撲通跪下來 srO vfk Orz

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
const long long INF = 1e18;
const int N = 50+5;
vector< pair<int, ll> > g[N], p[N];
int n, m, x, y, s, t, k;
ll dis[N], z;

void dijkstra() {
    for (int i = 1
 
; i <= n; i++) {
        dis[i] = INF;
    }
    dis[t] = 0;
    priority_queue< pair<ll, int> > q;
    q.push(make_pair(0, t));
    while (!q.empty()) {
        pair<ll, int> _ = q.top();
        q.pop();
        int u = _.second;
        for (int i = 0; i < p[u].size(); i++) {
            
 
int v = p[u][i].first;
            ll w = p[u][i].second;
            if (dis[v] > dis[u] + w) {
                dis[v] = dis[u] + w;
                q.push(make_pair(-dis[v], v));
            }
        }
    }
}

bool vis[N];
int way;
int sign[N];

bool run(int u, ll d, ll lim) {
    vis[u] = true
 
;
    if (u == t) {
        way += (d <= lim);
    } else {
        for (int i = 0; i < g[u].size(); i++) {
            int v = g[u][i].first;
            ll w = g[u][i].second;
            if (!vis[v] && d + w + dis[v] <= lim) {
                if (run(v, d + w, lim)) {
                    vis[u] = false;
                    return true;
                }
            }
        }
    }
    vis[u] = false;
    return way >= k;
}

ll ksp() {
    ll l = -2, r = INF;
    while (r - l > 1) {
        ll lim = (l + r) / 2;
        way = 0;
        if (run(s, 0, lim)) {
            r = lim;
        } else {
            l = lim;
        }
    }
    return r;
}

int fa[N];
int kk;

void dfs0(int u, ll d, ll tar) {
    if (u == t) {
        kk += (d < tar ? 1 : 0);
    }
    vis[u] = true;
    for (int i = 0; i < g[u].size(); i++) {
        int v = g[u][i].first;
        ll w = g[u][i].second;
        if (!vis[v] && d + w + dis[v] <= tar) {
            dfs0(v, d + w, tar);
        }
    }
    vis[u] = false;
}

bool dfs(int u, ll d, ll tar) {
    if (u == t) {
        k -= (d <= tar);
        if (d == tar) {
            return k == 0;
        }
        return false;
    }
    vis[u] = true;
    for (int i = 0; i < g[u].size(); i++) {
        int v = g[u][i].first;
        ll w = g[u][i].second;
        if (!vis[v] && d + w + dis[v] <= tar) {
            fa[v] = u;
            if (dfs(v, d + w, tar)) {
                return true;
            }
        }
    }
    return vis[u] = false;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0); cout.tie(0);
    cin >> n >> m >> k >> s >> t;
    for (int i = 1; i <= m; i++) {
        cin >> x >> y >> z;
        g[x].push_back(make_pair(y, z));
        p[y].push_back(make_pair(x, z));
    }
    dijkstra();
    puts("");
    ll tar = ksp();
    if (tar == -1 || tar == INF) {
        cout << "No" << endl;
    } else {
        for (int i = 1; i <= n; i++) {
            vis[i] = false;
        }
        dfs0(s, 0, tar);
        k -= kk;
        assert(dfs(s, 0, tar));
        vector<int> ans;
        for (int i = t; i != s; i = fa[i]) {
            ans.push_back(i);
        }
        reverse(ans.begin(), ans.end());
        cout << s;
        for (int i = 0; i < ans.size(); i++) {
            cout << ‘-‘ << ans[i];
        }
        puts("");
    }
    return 0;
}

（有一個數據是打表過的(￣▽￣)這裏沒放出來，否則太醜了(′?_?`)）

BZOJ 1073: [SCOI2007]kshort