Two Pointer with Filtering

Filtering removes elements matching a rule while scanning once: move valid elements to the front with a write pointer.

Try this in Python

def remove_value(nums: list[int], val: int) -> int:
    w = 0
    for x in nums:
        if x != val:
            nums[w] = x
            w += 1
    return w


a = [3, 2, 2, 3]
k = remove_value(a, 3)
print(k, a[:k])

Key takeaways

• In-place filtering uses read/write pointers.
• Stable order preserved here.
• Return new logical length.