Submission #24530

#	Time	Username	Problem	Language	Result	Execution time	Memory
24530		Bruteforceman	Network (BOI15_net)	C++11	100 / 100	1116 ms	52880 KiB

net

#include "bits/stdc++.h"
using namespace std;
int deg[500010];
deque <int> nodes; 
vector <int> g[500010];
int sub[500010];
int parent[500010];

void leaf(int x, int par) {
	if(deg[x] == 1) sub[x] = 1;
	else sub[x] = 0;
	for(auto i : g[x]) {
		if(i - par) {
			leaf(i, x);
			sub[x] += sub[i];
		}
	}
}

int centroid(int x, int par, int range) {
	for(auto i : g[x]) {
		if(i - par) {
			if(sub[i] > range) {
				return centroid(i, x, range);                                                                                                             
			}
		}
	}
	return x;
}

void dfs(int x, int par) {
	parent[x] = par;
	if(deg[x] == 1) {
		nodes.push_back(x);
	}
	for(auto i : g[x]) {
		if(i - par) {
			dfs(i, x);		
		} 
	}
}

bool vis[500010];
int lca(int x, int y) {
	memset(vis, false, sizeof vis);
	while(x != 0) {
		vis[x] = true;
		x = parent[x];
	}
	while(y != 0) {
		if(vis[y] == true) return y;
		y = parent[y];
	}
	return 1;
}


int main(int argc, char const *argv[])
{
	int n;
	scanf("%d", &n);

	for(int i = 1; i < n; i++) {
		int p, q;
		scanf("%d %d", &p, &q);
		++deg[p];
		++deg[q];
		g[p].push_back(q);
		g[q].push_back(p);
	}

	int root = 0;
	for(int i = 1; i <= n; i++) {
		if(deg[i] != 1) {
			root = i;
			break;
		}
	}
	leaf(root, 0);
	int half = (sub[root] + 1) >> 1;
	int leave = sub[root];


	int centre = centroid(root, 0, half);
	dfs(centre, 0);
	leaf(centre, 0);

	assert(deg[centre] != 1);
	for(auto i : g[centre]) {
		if(sub[i] > half) {
			assert(false);
		}
 	}

	printf("%d\n", half);
	for(int i = 0; i + half < leave; i++) {
		printf("%d %d\n", nodes[i], nodes[i + half]);
	}
	if(leave & 1) {
		if(lca(nodes[half-1], nodes.back()) == centre) printf("%d %d\n", nodes[half-1], nodes.back());
		else printf("%d %d\n", nodes[half-1], nodes.front());
	}
	return 0;
}

Compilation message (stderr)

net.cpp: In function 'int main(int, const char**)':
net.cpp:61:17: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d", &n);
                 ^
net.cpp:65:25: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d %d", &p, &q);
                         ^

• Subtask 1: 18 / 18
• Subtask 2: 45 / 45
• Subtask 3: 37 / 37