func_code_string: #include <bits/stdc++.h>   using namespace std;   const long long mod = 1e9 + 7;   const int N = 7e4;   const int INF = 1e9;   struct d {     int ad, xr;   } a[N];   stack<int> s;   vector<pair<int, int>> v;   int n;   int main() {     ios::sync_with_stdio(0);     cin.tie();     cout.tie();     cin >> n;     int A, B;     for (int i = 0; i < n; ++i) {       cin >> A >> B;       if (A == 0) continue;       if (A == 1) s.push(i);       a[i] = {A, B};     }     while (!s.empty()) {       int x = s.top();       s.pop();       if (a[x].ad == 1) {         v.push_back({x, a[x].xr});         a[x].ad--;         --a[a[x].xr].ad;         a[a[x].xr].xr ^= x;         if (a[a[x].xr].ad == 1) s.push(a[x].xr);       }     }     cout << v.size() <<   n ;     for (auto x : v) cout << x.first <<     << x.second <<   n ;   }   