有向圖找負環 (Negative-Cycle)
CSES - 1197 - Cycle Finding
Codeforces GYM 100274
2013-2014 CT S01E09 --- GCPC 2011 + GCJ 2009 R3 C-D
Codeforces Round #615 (Div. 3)
Codeforces Round
