Two Sum II - Input Array Is Sorted (LC 167)

Given a 1-indexed array of integers numbers that is already sorted in non-decreasing order, find two numbers such that they add up to a specific target number. Let these two numbers be numbers[index1] and numbers[index2] where 1 <= index1 < index2 <= numbers.length.

Return the indices of the two numbers, index1 and index2, added by one as an integer array [index1, index2] of length 2.

The tests are generated such that there is exactly one solution. You may not use the same element twice.

Your solution must use only constant extra space.

Solution

def twoSum(self, numbers: List[int], target: int) -> List[int]:
	
	l, r = 0, len(numbers)-1
	
	while l < r:
		currentSum = numbers[l] + numbers[r]
		
		if currentSum < target:
			l += 1
			
		elif currentSum > target:
			r -= 1
		
		else:
			return ([l+1, r+1]) # Adjust for 1-index

• Pretty straightforward 2-pointers approach
• Increment left pointer if we need bigger number
• Decrement right pointer if we need smaller number
• One thing to note here is that we have to use elif for the second check to make sure the two cases are mutually exclusive. If we use if for both conditions, there’s a potential risk of modifying both l and r in the same iteration.