Factorial

大家有冇試過係google search "1!" 呢
佢會計左個 "1"出黎，究竟個感嘆號"!"係咩黎呢？
其實佢係個數學運算符號，個做法叫Factorial
n!=n(n-1)(n-2)....3‧2‧1 or defined as

後面果舊野解作For All n, n is an element of Natural Number
即係話n要係個自然數，0,1,2,3,4,5,6果D，或者Non-negative Integer
而0!=1

Factorial 係好重要既野，遲下再講