#include <cstdio> #include <vector> #define MAXN 500000 std::vector < int > g[MAXN + 1], v; int s[MAXN + 1], cost[MAXN + 1]; bool viz[MAXN + 1], done[MAXN + 1]; void dfs(int x) { s[x] = g[x].size() == 1; viz[x] = 1; for (auto &y : g[x]) { if (viz[y] == 0) { dfs(y); s[x] += s[y]; cost[x] = std::max(cost[x], s[y]); } } } void dfs2(int x) { if (g[x].size() == 1) v.push_back(x); done[x] = 1; for (auto &y : g[x]) if (done[y] == 0) dfs2(y); } int main() { int n; scanf("%d", &n); for (int i = 1; i < n; i++) { int x, y; scanf("%d%d", &x, &y); g[x].push_back(y); g[y].push_back(x); } dfs(1); int ans = n + 1, r = 0; for (int i = 1; i <= n; i++) { if (g[i].size() != 1) { int val = std::max(cost[i], s[1] - s[i]); if (val < ans) ans = val, r = i; } } printf("%d\n", (s[1] + 1) / 2); dfs2(r); int k = v.size() / 2; for (int i = 0; i < k; i++) printf("%d %d\n", v[i], v[i + k]); if (v.size() % 2) printf("%d %d\n", v.back(), v[0]); return 0; }

net.cpp: In function 'int main()':
net.cpp:33:10: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d", &n);
     ~~~~~^~~~~~~~~~
net.cpp:37:14: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
         scanf("%d%d", &x, &y);
         ~~~~~^~~~~~~~~~~~~~~~