Codeforces 238E Meeting He （看题解）

Meeting He

感觉对这种题好无力啊， 以后这种感觉没有办法一次性dp完成的可以考虑用类似于bellman的方法来求最优值。

#include<bits/stdc++.h>
#define LL long long
#define LD long double
#define ull unsigned long long
#define fi first
#define se second
#define mk make_pair
#define PLL pair<LL, LL>
#define PLI pair<LL, int>
#define PII pair<int, int>
#define SZ(x) ((int)x.size())
#define ALL(x) (x).begin(), (x).end()
#define fio ios::sync_with_stdio(false); cin.tie(0);

using namespace std;

const int N = 100 + 7;
const int inf = 0x3f3f3f3f;
const LL INF = 0x3f3f3f3f3f3f3f3f;
const int mod = 1e9 + 7;
const double eps = 1e-8;
const double PI = acos(-1);

template<class T, class S> inline void add(T &a, S b) {a += b; if(a >= mod) a -= mod;}
template<class T, class S> inline void sub(T &a, S b) {a -= b; if(a < 0) a += mod;}
template<class T, class S> inline bool chkmax(T &a, S b) {return a < b ? a = b, true : false;}
template<class T, class S> inline bool chkmin(T &a, S b) {return a > b ? a = b, true : false;}

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

int n, m, a, b, k, S[N], T[N];
int G[N][N], d[N][N], ans[N], dp[N];
int fnow;
int vis[N];
vector<int> V[N];
vector<PII> vc;


int dfs(int u, int who) {
    if(vis[u] == fnow) return dp[u];
    vis[u] = fnow;
    int ret = -1;
    for(int v = 1; v <= n; v++) {
        if(G[u][v] == 1 && d[S[who]][u] + d[v][T[who]] + 1 == d[S[who]][T[who]]) {
            chkmax(ret, dfs(v, who));
        }
    }
    if(ret == -1) ret = inf;
    chkmin(ret, ans[u]);
    return dp[u] = ret;
}

int main() {

    memset(ans, 0x3f, sizeof(ans));
    memset(G, 0x3f, sizeof(G));
    memset(d, 0x3f, sizeof(d));

    scanf("%d%d%d%d", &n, &m, &a, &b);
    for(int i = 1; i <= n; i++) {
        G[i][i] = d[i][i] = 0;
    }
    for(int i = 1; i <= m; i++) {
        int u, v;
        scanf("%d%d", &u, &v);
        G[u][v] = d[u][v] = 1;
    }

    for(int k = 1; k <= n; k++) {
        for(int i = 1; i <= n; i++) {
            for(int j = 1; j <= n; j++) {
                chkmin(d[i][j], d[i][k] + d[k][j]);
            }
        }
    }

    scanf("%d", &k);
    for(int o = 1; o <= k; o++) {
        int s, t;
        scanf("%d%d", &s, &t);
        S[o] = s; T[o] = t;
        vc.clear();
        if(d[s][t] >= inf) continue;
        for(int i = 1; i <= n; i++) {
            for(int j = 1; j <= n; j++) {
                if(G[i][j] != 1) continue;
                if(d[s][i] + d[j][t] + 1 == d[s][t]) {
                    vc.push_back(mk(d[s][i], i));
                    vc.push_back(mk(d[s][j], j));
                }
            }
        }
        sort(ALL(vc));
        vc.erase(unique(ALL(vc)), vc.end());
        for(int i = 0; i < SZ(vc); i++) {
            if(i && vc[i].fi == vc[i - 1].fi) continue;
            if(i < SZ(vc) - 1 && vc[i].fi == vc[i + 1].fi) continue;
            V[o].push_back(vc[i].se);
        }
    }

    ans[b] = 0;

    while(1) {
        bool flag = false;
        for(int o = 1; o <= k; o++) {
            if(d[S[o]][T[o]] >= inf) continue;
            for(auto &u : V[o]) {
                fnow++;
                int ret = dfs(u, o) + 1;
                if(chkmin(ans[u], ret)) {
                    flag = true;
                }
            }
        }
        if(!flag) break;
    }
    printf("%d
", ans[a] < inf ? ans[a] : -1);

    return 0;
}

/*
*/