What is nPr ??

Permutations(排列)
Each of the arrangement which can be made by taking some or all of a number of given objects is called a permutation.
The number of different permutations of n different objects taken r at a time is denoted by nPr.
Consider the number of ways of filling r boxes with objects taken from a list of n given objects.
The number of ways=n(n-1)(n-2)(n-3)......(n-r+1)=n!/(n-r)!

nPr=n!/r!
n個ball揪r個ball出來
但是,
{1,2,3}, {1,3,2},{3,2,1}
是different case