Function egcd

code/mathematics/egcd.cpp:1–4

ll egcd(ll a, ll b, ll& x, ll& y) {
  if (b == 0) { x = 1; y = 0; return a; }
  ll d = egcd(b, a % b, x, y);
  x -= a / b * y; swap(x, y); return d; }
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