Move Zeroes Using Two Pointers

Problem Statement

Given an array nums, move all 0s to the end while keeping the relative order of the non-zero elements. Do it in place without making a copy.

Example

Input:  nums = [0,1,0,3,12]
Output: [1,3,12,0,0]

Two Pointer Approach

Keep a last write pointer for the next non-zero slot. Scan with a read pointer; each time you see a non-zero value, write it at last and increment last. Finally, fill the remaining positions with zeroes.

Complexity

• Time: O(n)
• Space: O(1)

Solution Code

Python

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

Java

void moveZeroes(int[] nums) {
    int last = 0;
    for (int i = 0; i < nums.length; i++) {
        if (nums[i] != 0) nums[last++] = nums[i];
    }
    while (last < nums.length) nums[last++] = 0;
}

C++

void moveZeroes(vector<int>& nums) {
    int last = 0;
    for (int i = 0; i < (int)nums.size(); i++) {
        if (nums[i] != 0) nums[last++] = nums[i];
    }
    while (last < (int)nums.size()) nums[last++] = 0;
}

Key Takeaways

• Order is preserved because we copy non-zeros left to right.
• A single swap-based variant also works and avoids the second loop.
• This is the same slow/fast write pattern as Remove Element.