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Move Zeroes – Solution & Complexity

Interview solutions
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Move Zeroes
Quick practice for Move Zeroes: 3 reasoning beats, no typing

Solution Walkthrough

  1. Step 1: Understand the Goal

    • Move every 0 to the end of the array.
    • Preserve the relative order of all non-zero values.
    • The function returns the resulting array.

  2. Step 2: Simple Extra-Array Approach

    • Collect all non-zero values first.
    • Count how many zeroes are needed at the end.
    • This is easy to understand but uses extra space.
    def move_zeroes(nums):
    non_zeroes = [num for num in nums if num != 0]
    zero_count = len(nums) - len(non_zeroes)
    return non_zeroes + [0] * zero_count

  3. Step 3: Use a Write Pointer

    • Keep a pointer for where the next non-zero value should be placed.
    • Scan the array from left to right.
    • Every non-zero value is written to the next available position, preserving order.

  4. Step 4: Fill the Remaining Positions with Zeroes

    • After all non-zero values are written, everything from the write pointer to the end should be 0.
    • This produces the same array shape without needing a second array.
    • Finally, return nums.
    def move_zeroes(nums):
    write = 0
    for num in nums:
        if num != 0:
            nums[write] = num
            write += 1
    for i in range(write, len(nums)):
        nums[i] = 0
    return nums

  5. Step 5: Final Solution and Complexity

    • The solution keeps non-zero elements in their original relative order.
    • Time complexity is O(n).
    • Space complexity is O(1) besides the input array.
    def move_zeroes(nums: list[int]) -> list[int]:
    write = 0
    for num in nums:
        if num != 0:
            nums[write] = num
            write += 1
    for i in range(write, len(nums)):
        nums[i] = 0
    return nums
Learn the Two Pointers pattern: recognition cues, invariant, and ordered practice →

FAQ

See also

Practice Move Zeroes online (run code in the browser)3Sum solution (medium)3Sum Closest solution (medium)Container With Most Water solution (medium)Find the Duplicate Number solution (medium)All coding interview problems
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