Fisher–Yates洗牌算法

To initialize an array a of n elements to a randomly shuffled copy of source, both 0-based:
  a[0] ← source[0]
  for i from 1 to n − 1 do
      j ← random integer with 0 ≤ j ≤ i
      a[i] ← a[j]
      a[j] ← source[i]

相关：http://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle

http://coolshell.cn/articles/8593.html