#include<bits/stdc++.h>
#define STIZE(x) fprintf(stderr, "STIZE%d\n", x);
#define PRINT(x) fprintf(stderr, "%s = %d\n", #x, x);
#define NL(x) printf("%c", " \n"[(x)]);
#define lld long long
#define pii pair<int,int>
#define pb push_back
#define fi first
#define se second
#define all(a) begin(a),end(a)
#define sz(a) int((a).size())
#define LINF LLONG_MAX
#define INF INT_MAX
#define EPS 1e-9
using namespace std;
#define MAXN 200010
int n, root, centroid;
int deg[MAXN];
int brlist[MAXN], cntLeaf;
bool is_leaf[MAXN];
vector<int> adj[MAXN];
int dfs1(int cur, int pret) {
if(brlist[cur] != 0) return brlist[cur];
if(is_leaf[cur]) return brlist[cur] = 1;
for(auto x : adj[cur]) {
if(x != pret) brlist[cur] += dfs1(x, cur);
}
return brlist[cur];
}
int dfs2(int cur, int pret) {
int maxi = 0;
for(auto x : adj[cur]) {
if(!is_leaf[x] && brlist[x] >= brlist[maxi] && x != pret) maxi = x;
}
if(brlist[maxi] > cntLeaf-cntLeaf/2) return dfs2(maxi, cur);
else return cur;
}
int susedi;
vector<int> lisce[MAXN];
void dfs3(int cur, int pret) {
if(is_leaf[cur]) {
lisce[susedi].pb(cur);
return;
}
for(auto x : adj[cur]) {
if(x != pret) dfs3(x, cur);
if(cur == centroid) susedi++;
}
}
priority_queue<pii> pq;
int main() {
scanf("%d", &n);
if(n <= 2) return printf("1\n1 %d", n), 0;
for(int i = 0; i < n-1; i++) {
int x, y; scanf("%d%d", &x, &y);
adj[x].pb(y);
adj[y].pb(x);
deg[x]++; deg[y]++;
}
for(int i = 1; i <= n; i++) {
if(deg[i] == 1) is_leaf[i] = 1, cntLeaf++;
else root = i;
}
dfs1(root, 0);
centroid = dfs2(root, 0);
dfs3(centroid, 0);
for(int i = 0; i < susedi; i++) {
pq.push({sz(lisce[i]), i});
}
printf("%d\n", sz(leaf)-sz(leaf)/2);
while(sz(pq) > 1) {
pii vrh1 = pq.top(); pq.pop();
pii vrh2 = pq.top(); pq.pop();
int komp1 = vrh1.se, komp2 = vrh2.se;
int rep1 = vrh1.fi, rep2 = vrh2.fi;
printf("%d %d\n", lisce[komp1][--rep1], lisce[komp2][--rep2]);
if(rep1) pq.push({rep1, komp1});
if(rep2) pq.push({rep2, komp2});
}
if(pq.size() == 1) {
int komp = pq.top().se;
printf("%d %d\n", lisce[komp][0], centroid);
}
return 0;
}
Compilation message
net.cpp: In function 'int main()':
net.cpp:70:23: error: 'leaf' was not declared in this scope
printf("%d\n", sz(leaf)-sz(leaf)/2);
^
net.cpp:11:20: note: in definition of macro 'sz'
#define sz(a) int((a).size())
^
net.cpp:52:20: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
scanf("%d", &n);
^
net.cpp:55:40: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
int x, y; scanf("%d%d", &x, &y);
^
