When to Use

Look for these signals in the problem:

• "Reverse" a list or portion: Pointer manipulation reverses in-place without extra space.
• "Detect cycle" in sequence: Fast and slow pointers (Floyd's algorithm) detect cycles in $O(1)$ space.
• "Find middle" element: Two-pointer technique finds middle in single pass.
• "Merge sorted" lists: Linked list merge is simpler than array merge (no shifting).
• "Remove nth from end": Two pointers with n-gap find the target in one pass.