func_code_string: #include <bits/stdc++.h>   using namespace std;   struct ji {     int nex, to;   } edge[100005 << 1];   set<pair<int, int> > s[100005];   set<pair<int, int> >::iterator it;   priority_queue<pair<int, int> > q;   pair<int, int> ans[100005];   int E, n, r, x, y, z, cnt, head[100005], sz[100005], mx[100005], f[100005],       d[100005];   long long m;   void add(int x, int y) {     edge[E].nex = head[x];     edge[E].to = y;     head[x] = E++;   }   void dfs(int k, int fa) {     sz[k] = 1;     mx[k] = 0;     for (int i = head[k]; i != -1; i = edge[i].nex)       if (edge[i].to != fa) {         dfs(edge[i].to, k);         sz[k] += sz[edge[i].to];         mx[k] = max(mx[k], sz[edge[i].to]);       }     if (max(n - sz[k], mx[k]) <= n / 2) r = k;   }   void dfs(int k, int fa, int sh) {     f[k] = fa;     d[k] = sh;     m -= sh;     s[x].insert(make_pair(sh, k));     sz[k] = 1;     for (int i = head[k]; i != -1; i = edge[i].nex)       if (edge[i].to != fa) {         dfs(edge[i].to, k, sh + 1);         sz[k] += sz[edge[i].to];       }   }   int main() {     scanf( %d%lld , &n, &m);     memset(head, -1, sizeof(head));     for (int i = 1; i < n; i++) {       scanf( %d%d , &x, &y);       add(x, y);       add(y, x);     }     dfs(1, 0);     for (int i = head[r]; i != -1; i = edge[i].nex) {       x = edge[i].to;       dfs(x, r, 1);       q.push(make_pair(sz[x], x));     }     if ((m > 0) || (m % 2)) {       printf( NO );       return 0;     }     m /= -2;     while (m) {       x = q.top().second;       q.pop();       if (sz[x] < 2) {         printf( NO );         return 0;       }       sz[x] -= 2;       if (sz[x]) q.push(make_pair(sz[x], x));       if ((*--s[x].end()).first - 1 <= m) {         y = (*--s[x].end()).second;         s[x].erase(--s[x].end());         m -= d[y] - 1;         z = f[y];         bool flag = 0;         for (int i = head[z]; i != -1; i = edge[i].nex)           if ((edge[i].to != f[z]) && (edge[i].to != y) &&               (s[x].find(make_pair(d[edge[i].to], edge[i].to)) != s[x].end())) {             ans[++cnt] = make_pair(edge[i].to, y);             s[x].erase(make_pair(d[edge[i].to], edge[i].to));             flag = 1;             break;           }         if (!flag) {           ans[++cnt] = make_pair(z, y);           s[x].erase(make_pair(d[z], z));         }       } else {         it = lower_bound(s[x].begin(), s[x].end(), make_pair((int)m, 0));         while ((*it).first == m) {           bool flag = 0;           y = (*it).second;           for (int i = head[y]; i != -1; i = edge[i].nex)             if ((edge[i].to != f[y]) &&                 (s[x].find(make_pair(d[edge[i].to], edge[i].to)) != s[x].end())) {               ans[++cnt] = make_pair(y, edge[i].to);               s[x].erase(make_pair(d[y], y));               s[x].erase(make_pair(d[edge[i].to], edge[i].to));               flag = 1;               break;             }           if (flag) break;           it++;         }         m = 0;       }     }     while (!q.empty()) {       x = q.top().second;       q.pop();       if (q.empty()) {         ans[++cnt] = make_pair((*s[x].begin()).second, r);         break;       }       y = q.top().second;       q.pop();       if (sz[x] > 1) q.push(make_pair(--sz[x], x));       if (sz[y] > 1) q.push(make_pair(--sz[y], y));       ans[++cnt] = make_pair((*s[x].begin()).second, (*s[y].begin()).second);       s[x].erase(s[x].begin());       s[y].erase(s[y].begin());     }     printf( YES n );     for (int i = 1; i <= cnt; i++) printf( %d %d n , ans[i].first, ans[i].second);   }   