Move Zeroes

Given an integer array nums, move all 0's to the end while maintaining the relative order of the non-zero elements. You must do this in-place without making a copy of the array. Minimize the total number of operations.

• All non-zero elements should appear in the same relative order as in the original array. All zeros should be at the end.
• We can do a single pass with a "write" pointer: whenever we see a non-zero, place it at the write position and advance write. Then fill the rest with zeros (or swap to avoid a second pass).

• Two pointers: write points to the next position where we should put a non-zero. We scan with read. When nums[read] != 0, we put it at write (swap with nums[write] or just assign nums[write] = nums[read]) and write++. After the scan, all non-zeros are in [0..write-1] in order; we can set nums[write..n-1] = 0 if we used assign instead of swap.
• If we swap, we don't need a second loop: each non-zero is swapped to the front and the zero that was there (if any) goes to the current read position and will be processed later.

High-level: write = 0. For each index read: if nums[read] != 0, swap nums[write] and nums[read] (or assign nums[write]=nums[read] and later zero out the tail), then write++. All non-zeros end up in [0..write-1] in order.

Steps:

1. write = 0. For read = 0..n-1: if nums[read] != 0, swap nums[write] and nums[read], write++.
2. (Optional) For i = write..n-1, set nums[i] = 0 if we used assign instead of swap.

• Time: O(n).
• Space: O(1).

• Move zeros to the front; move a specific value to end.