排列组合的计算，计算所有组合方式
例如：从5个中选出3个所有的组合
Permutation * p = [Permutation new];
NSArray * indexArrays = [p pickPermutation:3 totalNum:5];
"0,1,2",
"0,1,3",
"0,1,4",
"0,2,3",
"0,2,4",
"0,3,4",
"1,2,3",
"1,2,4",
"1,3,4",
"2,3,4"
NSInteger count = [p pickNum:pickNum totalNum:totalNum];