Remove Duplicates from Sorted Array in O(1) Space

Question

Question: https://leetcode.com/problems/remove-duplicates-from-sorted-array/description/

Basically, duplicates in a sorted array needs to be removed without creating a new one. I have tested my solution, and it falters when there are 3, 5, 7 (and so on) number of duplicates of the same number. What techniques can I try?

Code:

def removeDuplicates(nums):
    i = 0 # moving pointer
    end = len(nums)-1 # points to the end of the array
    # iterates over the array
    while i != end:
        # if current number is equal to the next
        if (nums[i] == nums[i+1]):
            # pop it off
            nums.pop(i)
            # decrement the length of array
            end -= 1
        i += 1 # increments the moving pointer
    return i + 1


nums = [0,0,1,1,1,2,2,3,3,4]
print(removeDuplicates(nums))

Asked By: Rhodey

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Source

Answer 1

The way to remove duplicates from a sorted list in O(1) space and O(n) time is to pack the unique elements at the beginning of the list, and then truncate the list:

def removeDuplicates(nums):
    #new length
    newlen = 0
    for val in nums:
        if newlen == 0 or nums[newlen-1] != val:
            # no matter what, we've already retrieved nums[newlen],
            # so this write doesn't interfere with the rest of the loop
            nums[newlen] = val
            newlen += 1
    
    # delete the unused space at the end of the list, all at once
    if newlen < len(nums):
        del nums[newlen:]
    return newlen


nums = [0,0,1,1,1,2,2,3,3,4]
print(removeDuplicates(nums))
print(nums)

Your original code fails, because you remove elements from the start of the array as you’re iterating through the end. The iteration remembers its position in the array in i, and when you remove an element, the elements after it are shifted into lower positions.

You could fix that by putting the i += 1 into an else, but the resulting implementation would still take O(N^2) time, which isn’t really acceptable.

Answered By: Matt Timmermans

Remove Duplicates from Sorted Array in O(1) Space

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