From f86e01c8e4b1ef9725e0be102cf573d1359080e3 Mon Sep 17 00:00:00 2001
From: Ariel <ariel@arielherself.xyz>
Date: Mon, 19 Feb 2024 16:53:47 +0800
Subject: [PATCH] Update exgcd.cc

---
 number/exgcd.cc | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/number/exgcd.cc b/number/exgcd.cc
index c1d276d..fdda090 100644
--- a/number/exgcd.cc
+++ b/number/exgcd.cc
@@ -50,6 +50,10 @@ namespace Exgcd {
 
     // find the minimal non-negative integral solution of `ax = b (mod n)`
     optional<ll> congruential(ll a, ll b, ll n) {
+        if (a == 0) {
+            if (b != 0) return nullopt;
+            return 0;
+        }
         if (a < 0 && a != LLONG_MIN && b != LLONG_MIN) a = -a, b = -b;
         auto sol = diophantine(a, n, b);
         if (sol == nullopt) {