Coding Challenge — 1

This problem was recently asked by Google

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]
def sortNums(nums):
  # Fill this in.
print sortNums([3, 3, 2, 1, 3, 2, 1])
# [1, 1, 2, 2, 3, 3, 3]

Challenge: Try sorting the list using constant space.

Solution1:

def sortNums(nums):
    #print(nums)
    a = 0
    b = 0
    c = 0
for i in nums:
        if i == 1:
            a += 1
        elif i == 2:
            b += 1
        else:
            c += 1
    
    return [1] * a + [2] * b + [3] * c
print(sortNums([3, 3, 2, 1, 3, 2, 1]))
# [1, 1, 2, 2, 3, 3, 3]

Solution2:

The time complexity of O(n), the space complexity is O(1)

def sortNums(nums):
    one_idx = 0
    three_idx = len(nums) - 1
    idx = 0
while idx < three_idx:
        if nums[idx] == 1:
            nums[idx], nums[one_idx] = nums[one_idx], nums[idx]
            idx += 1
            one_idx += 1
        elif nums[idx] == 2:
            idx += 1
        else:
            nums[idx], nums[three_idx] = nums[three_idx], nums[idx]
            three_idx -= 1
    
    return nums
print(sortNums([3, 3, 2, 1, 3, 2, 1]))
# [1, 1, 2, 2, 3, 3, 3]