Move Zeros

#algorithms #beginners #interview #python

Move Zeroes

Problem

You’re given an array and need to move all 0s to the end, while keeping the order of the non-zero elements the same.

The catch is that it has to be done in-place.

Strategy

At first, it feels like you need to shift elements every time you see a zero.

But that quickly gets messy.

Instead, I focused on the non-zero elements:

Keep a position for where the next non-zero should go
Traverse the array once
Every time I see a non-zero, move it to that position

So instead of pushing zeros back, I’m just pulling non-zero elements forward.

Code

class Solution:
    def moveZeroes(self, nums):
        j = 0

        for i in range(len(nums)):
            if nums[i] != 0:
                nums[j], nums[i] = nums[i], nums[j]
                j += 1

Key Lines Explained

j = 0
This keeps track of where the next non-zero element should be placed.
if nums[i] != 0:
We only act when we find a non-zero value.
nums[j], nums[i] = nums[i], nums[j]
This moves the non-zero element forward without losing order.
j += 1
Move to the next position after placing a non-zero.

Why This Works

All non-zero elements are moved forward in the same order they appear.

Zeros are never explicitly moved — they just get pushed to the end as a result of the swaps.

Complexity

Time: O(n)
Space: O(1)

Final Note

This problem looks like it’s about zeros, but it’s actually about placing non-zero elements correctly.

Once that perspective shifts, the solution becomes straightforward.

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