Using Two Pointers

def twoPointers(array: list[int], target: int) -> bool:
    left = 0
    right = len(array) - 1

    while left < right:
        answer = array[left] + array[right]
        
        if answer == target:
            return True
        elif answer > target:
            right -= 1
        elif answer < target:
            left += 1

    return False  

Code Explanation

Initialize Pointers:

left = 0
right = len(array) - 1

• left is initialized to the first index of the array (0).

• right is initialized to the last index of the array (len(array) - 1).

Two-Pointer Approach:

while left < right:
    answer = array[left] + array[right]

• The loop continues as long as left is less than right. This condition ensures that the two pointers do not cross and that distinct elements are considered.

Check the Sum (answer):

if answer == target:
    return True

• Calculate the sum (answer) of the elements at the left and right pointers.

• If answer equals the target, the function returns True, indicating that a pair with the desired sum has been found.

Adjust Pointers Based on the Comparison:

elif answer > target:
    right -= 1
elif answer < target:
    left += 1

• If answer is greater than target, decrement the right pointer (right -= 1). This moves the right pointer to a smaller element, reducing the sum.

• If answer is less than target, increment the left pointer (left += 1). This moves the left pointer to a larger element, increasing the sum.

How It Works: The two-pointer method works by leveraging the sorted nature of the array. By adjusting the pointers (left, right) based on the current sum (answer), it efficiently narrows down the search space for finding the desired pair.

This technique ensures that all potential pairs are considered without redundancy.

Connected Readings

• When to use a Two Pointer Approach by Geeksforgeeks (Link)