Two Pointers

Two indices scanning a sequence — converging from the ends, or one chasing the other.

Core syntax

• Converging — l, r = 0, len(a) - 1, move inward while l < r.
• Skip filter — while l < r and not a[l].isalnum(): l += 1.
• Pair iteration — for x, y in zip(a, a[1:]).

l, r = 0, len(s) - 1
while l < r:
    if s[l] != s[r]:
        return False
    l, r = l + 1, r - 1

Watch out

• Reverse a string/list with a slice: s[::-1].
• Sorted input is the usual precondition for the converging variant.