DSA — Two Pointers

What is Two Pointers?

Using two pointers to traverse a data structure, usually from different ends.

Two Sum (Sorted Array)

def two_sum_sorted(arr, target):
    left, right = 0, len(arr) - 1
    
    while left < right:
        current_sum = arr[left] + arr[right]
        if current_sum == target:
            return [left, right]
        elif current_sum < target:
            left += 1
        else:
            right -= 1
    
    return [-1, -1]

Remove Duplicates

def remove_duplicates(arr):
    if not arr:
        return 0
    
    slow = 0
    
    for fast in range(1, len(arr)):
        if arr[fast] != arr[slow]:
            slow += 1
            arr[slow] = arr[fast]
    
    return slow + 1

Merge Sorted Arrays

def merge_sorted(arr1, arr2):
    result = []
    i = j = 0
    
    while i < len(arr1) and j < len(arr2):
        if arr1[i] <= arr2[j]:
            result.append(arr1[i])
            i += 1
        else:
            result.append(arr2[j])
            j += 1
    
    result.extend(arr1[i:])
    result.extend(arr2[j:])
    return result

Patterns

Pattern	Description
Opposite ends	Start from both ends
Same direction	Slow and fast pointers
Merging	Merge two sorted sequences

Mini Practice

1. Two sum in sorted array
2. Remove duplicates
3. Merge sorted arrays
4. Check if palindrome

Up Next

Continue with Bit Manipulation — bitwise operations.