🧩 Python DSA Patterns — Two Pointers 👆👇🐍

The two pointers technique uses two indices that move through an array or string, often from opposite ends or at different speeds.

It helps reduce nested loops and optimizes many problems from O(n²) to O(n).

✨ When to Use

• When the problem involves pairs or subarrays.
• When the array/string is sorted or can be sorted.
• When we need to find relationships between two elements.

⚡ Advantages

• Simplifies problems that would normally require nested loops.
• Often reduces time complexity from quadratic to linear.
• Easy to implement with just a few variables.

📝 Example 1 — Sorted Array: Two Sum Problem

Given a sorted array and a target, find if two numbers add up to the target.

def two_sum_sorted(arr, target):
    left, right = 0, len(arr) - 1

    while left < right:
        current = arr[left] + arr[right]

        if current == target:
            return (left, right)
        elif current < target:
            left += 1
        else:
            right -= 1

    return None

# Example
print(two_sum_sorted([1, 2, 4, 6, 10], 8))  # Output: (1, 3) -> 2+6

📝 Example 2 — Removing Duplicates from Sorted Array

Compact the array in place so each element appears only once.

def remove_duplicates(arr):
    if not arr:
        return 0

    left = 0

    for right in range(1, len(arr)):
        if arr[right] != arr[left]:
            left += 1
            arr[left] = arr[right]

    return left + 1

# Example
nums = [1, 1, 2, 2, 3]
length = remove_duplicates(nums)
print(nums[:length])  # Output: [1, 2, 3]

📝 Example 3 — Valid Palindrome Check

Check if a string is a palindrome, ignoring non-alphanumeric characters.

def is_palindrome(s: str) -> bool:
    s = "".join(ch.lower() for ch in s if ch.isalnum())
    left, right = 0, len(s) - 1

    while left < right:
        if s[left] != s[right]:
            return False
        left += 1
        right -= 1

    return True

# Example
print(is_palindrome("A man, a plan, a canal: Panama"))  # Output: True

✅ Key Takeaways

• Use two pointers when working with pairs, palindromes, or deduplication.
• Works best on sorted arrays or linear scans.
• A simple yet powerful way to eliminate nested loops.

🏋️ Practice Problems

1. Pair with Target Sum (classic 2-sum in sorted array).
2. Remove Duplicates from Sorted Array.
3. Valid Palindrome (string cleaning + two pointers).
4. Container With Most Water (two pointers moving inward).