Submission #669665

#	Time	Username	Problem	Language	Result	Execution time	Memory
669665		ziduo	Mousetrap (CEOI17_mousetrap)	C++14	100 / 100	999 ms	217292 KiB

mousetrap

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#include <bits/stdc++.h>
 
            using namespace std;
 
            const int INF = INT32_MAX;
 
            int N, T, M, P[1000005], rem[1000005];
            int DP[1000005], deg[1000005], S[1000005];
            vector<int> adj[1000005], C[1000005];
 
            void dfs(int v, int p) {
                if(v == M) rem[v] = 1;
                for(auto u : adj[v]) {
                    if(u == p) continue;
                    dfs(u, v); P[u] = v;
                    if(rem[u]) rem[v] = 1;
                    else {
                        C[v].push_back(u); ++deg[v];
                    }
                }
                sort(C[v].begin(), C[v].end(), [](int A, int B) { return DP[A] > DP[B]; });
                if(C[v].size() <= 1) DP[v] = deg[v];
                else DP[v] = deg[v] + DP[C[v][1]];
            }
            void dfs(int v) {
                DP[v] += S[v];
                for(auto u : adj[v]) {
                    if(u == P[v]) continue;
                    S[u] = S[v] + deg[v]; dfs(u);
                }
            }
            bool possible(int v) {
                int cur = 0, dep = 0;
                for(int e = M; e != T; e = P[e]) {
                    ++dep; int k = 0;
                    for(auto u : C[e]) {
                        if(DP[u] + cur - k > v) ++cur, ++k;
                    }
                    if(cur > dep || cur > v)
                        return false;
                }
                return true;
            }
 
            int32_t main() {
                ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
                cin >> N >> T >> M;
                for(int l = 0; l < N - 1; l++) {
                    int U, V; cin >> U >> V;
                    adj[U].push_back(V);
                    adj[V].push_back(U);
                }
                dfs(T, T); deg[T] = 0; dfs(T);
                int lo = 0, hi = N, ans = -1;
                while(lo <= hi) {
                    int md = (lo + hi) / 2;
                    if(possible(md)) ans = md, hi = md - 1;
                    else lo = md + 1;
                }
                cout << ans << "\n";
                return 0;
            }

• Subtask 1: 20 / 20
• Subtask 2: 25 / 25
• Subtask 3: 20 / 20
• Subtask 4: 35 / 35