排序一個僅有3個惟一數字元素組成的列表

題目

Given a list of numbers with only 3 unique numbers (1, 2, 3), sort the list in O(n) time.

Example 1:
Input: [3, 3, 2, 1, 3, 2, 1]
Output: [1, 1, 2, 2, 3, 3, 3]

Challenge: Try sorting the list using constant space.

分析

遍歷一遍，計數每一個惟一元素出現的次數。而後根據計數生成排序好的結果數組。
時間複雜度 O(n).

代碼

def sortNums(nums):
  # count
  counts = [0, 0, 0]
  for n in nums:
    counts[n - 1] += 1

  # sort
  return [1] * counts[0] + [2] * counts[1] + [3] * counts[2]

print sortNums([3, 3, 2, 1, 3, 2, 1])
# [1, 1, 2, 2, 3, 3, 3]