principle:
Addition principle (and counting principle)
Multiplication principle (fractional counting principle)
Arrangement (A)
General arrangement:
A(n,m) =n*(n-1)*(n-2)*...*(n-m+1) =n! / (n-m)!
When n = m, the whole arrangement is: A (n, n) = n!
May be repeatedly arranged:
It is n ^ m
Incomplete different:
Macro, is the general arrangement, then remove n1!, N2!, ... nk!
Round arrangement:
Where each element distinct default: A (n, m) / m
Composition (C)
General combinations:
C(n,m) * A(m,m) =A(n,m)
C(n,m)=n! / ( m! *(n-m+1)! )
C(n,m)=C(n-1,m) +C(n-1,m-1)
May be repeated in combination:
https://blog.csdn.net/guoyangfan_/article/details/82825012
Problems divided into pellets: C (m + n-1, m)