Two Sum II – Input Array Is Sorted

Two Sum II – Input Array Is Sorted
Problem

Now the array is already sorted, and we must use constant extra space.

Example
numbers = [2,7,11,15]
target = 9
Output: [1,2]

(Note: Index starts from 1)

Best Approach – Two Pointer Technique

Since array is sorted:

If sum is too small → move left pointer
If sum is too large → move right pointer

Algorithm Steps
left = 0
right = n - 1
While left < right:
sum = numbers[left] + numbers[right]
If sum == target → return indices
If sum < target → left++
If sum > target → right--

Python Code
def twoSum(numbers, target):
left = 0
right = len(numbers) - 1

while left < right:
    s = numbers[left] + numbers[right]

    if s == target:
        return [left + 1, right + 1]

    elif s < target:
        left += 1
    else:
        right -= 1

Complexity
Type Complexity
Time O(n)
Space O(1)

Difference Between Both Problems
Feature Two Sum Two Sum II
Array Unsorted Sorted
Method Hash Map Two Pointer
Space O(n) O(1)
Index 0-based 1-based
Time O(n) O(n)