combination-and-permutation/README.MD

Combination or Permutation computation

# Combination and Permutation computation 组合和排列计算   
  
## 1. Combination 组合  
  
Every combination has **same probability**.  
每一种组合都有**相同的概率**。  
  
execute combination(3) 10000 times, we got:  
执行了10000次combination(3)，结果如下：  
  '[2,3]' => 1469,  
  '[1,2]' => 1370,  
  '[1,2,3]' => 1447,  
  '[3]' => 1400,  
  '[1,3]' => 1413,  
  '[1]' => 1471,  
  '[2]' => 1430  
  
## 2. Permutation 排列  
Obviously, permutations and combinations are different, and permutations with different numbers have **different probabilities**。  
很明显，排列跟组合不同，不同数量数字的排列具有**不同的概率**。  
  
execute permutation(3) 10000 times, we got:  
执行了10000次permutation(3)，结果如下：  
  '[3]' => 1417,  
  '[2,1,3]' => 264,  
  '[2,3]' => 671,  
  '[3,1,2]' => 267,  
  '[3,1]' => 708,  
  '[1,3]' => 698,  
  '[2]' => 1412,  
  '[2,3,1]' => 230,  
  '[3,2,1]' => 226,  
  '[1,2,3]' => 260,  
  '[3,2]' => 746,  
  '[1,2]' => 750,  
  '[1,3,2]' => 225,  
  '[1]' => 1414,  
  '[2,1]' => 727