Submission #846651

#	Time	Username	Problem	Language	Result	Execution time	Memory
846651		trandangquang	Torrent (COI16_torrent)	C++14	100 / 100	609 ms	25720 KiB

torrent

#include <bits/stdc++.h>
using namespace std;

#define pii pair<int, int>
#define fi first 
#define se second 
const int N = 3e5+5;

int n, a, b, trace[N], dp[N], ans = N;
vector <pii> path;
vector <int> g[N];

void dfs(int u, int p) {
    for(int v:g[u]) if(v != p) {
        dfs(v, u);
        trace[v] = u;
    }
}

void dfs2(int u, int p, int E) {
    vector <int> child;
    for(int v:g[u]) {
        if(v == p or (u==path[E].fi and v == path[E].se) or (u==path[E].se and v==path[E].fi)) continue;
        dfs2(v, u, E);
        child.emplace_back(dp[v]);
    }
    sort(child.begin(), child.end());
    for(int i = 0; i < child.size(); ++i) {
        dp[u] = max(dp[u], child[i] + (int)child.size() - i);
    }
}

int val(int v, int E) {
    dfs2(v, v, E);
    return dp[v];
}   

bool check(int numE) {
    memset(dp, 0, sizeof(dp));
    int vala = val(a, numE), valb = val(b, numE);
    ans = min(ans, max(vala, valb));
    return (val(a, numE) > val(b, numE));
}

void binary_search() {
    int left = 0, right = path.size()-1;
    while(left <= right) {
        int mid = (left + right) >> 1;
        if(check(mid)) {
            left = mid + 1; // near a
        }
        else 
            right = mid - 1; // near b
    }
    cout << ans << '\n';
}

signed main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);

    if(fopen("test.inp", "r")) freopen("test.inp", "r", stdin);

    cin >> n >> a >> b;
    for(int i = 1; i < n; ++i) {
        int u, v; cin >> u >> v;
        g[u].emplace_back(v);
        g[v].emplace_back(u);
    }

    dfs(a, a);

    int tmp = b;
    while(tmp != a) {
        path.emplace_back(tmp, trace[tmp]);
        tmp = trace[tmp];
    }

    binary_search();
}

Compilation message (stderr)

torrent.cpp: In function 'void dfs2(int, int, int)':
torrent.cpp:28:22: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   28 |     for(int i = 0; i < child.size(); ++i) {
      |                    ~~^~~~~~~~~~~~~~
torrent.cpp: In function 'int main()':
torrent.cpp:62:39: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   62 |     if(fopen("test.inp", "r")) freopen("test.inp", "r", stdin);
      |                                ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~

• Subtask 1: 31 / 31
• Subtask 2: 69 / 69