## What is it? Move all zeros in an array to the end while maintaining the relative order of non-zero elements. Done in-place in a single pass. ## How it works - Use a slow pointer `insertPos = 0` tracking where the next non-zero element should go - Iterate with `i` from 0 to n-1: - If `arr[i] != 0` → place it at `insertPos`, advance `insertPos` - After the loop, fill positions from `insertPos` to `n-1` with zeros **Alternative (swap-based):** - Swap `arr[i]` and `arr[insertPos]` whenever `arr[i] != 0`, then advance `insertPos` ## When to use - In-place array cleanup with a "gather non-matching elements" pattern - Template for "move all elements satisfying condition to one end" ## Key Points - O(n) time, O(1) space - Maintains relative order of non-zero elements (stable) - The fill step is necessary in the copy approach; the swap approach avoids it but does more writes