Two Pointers Templates

Template 1: Converging (Opposite Ends)

Used for sorted arrays, palindromes, and Container With Most Water.

def two_pointers_converging(nums):
    left = 0
    right = len(nums) - 1

    while left < right: # STRICTLY less than to avoid comparing an element to itself
        current_val = nums[left] + nums[right]

        if current_val == target:
            return True
        elif current_val < target:
            # Need a larger value
            left += 1
        else:
            # Need a smaller value
            right -= 1

    return False

Template 2: Fast & Slow (Same Direction)

Used for Linked List cycle detection, finding the middle, and in-place array modifications (removing duplicates).

def fast_slow_pointers(head):
    slow = head
    fast = head

    # Must check both fast and fast.next to avoid AttributeError
    while fast and fast.next:
        slow = slow.next          # Move 1 step
        fast = fast.next.next     # Move 2 steps

        if slow == fast:
            return True # Cycle detected

    # If loop finishes, fast reached the end
    return False 

Crucial Reminders

1. Converging: Always use while left < right:. Do not use <=, or you will process the middle element twice or compare it to itself.
2. Fast/Slow: Always guard the loop with while fast and fast.next:. If you only check fast, fast.next.next will crash when fast.next is None.