0167.Two Sum II

Link: 167. Two Sum II - Input Array Is Sorted - Medium
Track: NeetCode150

Question

Restate the problem

• Given a sorted integer array numbers and an integer target
• Find two numbers such that they add up to target
• Return their indices as 1-indexed: [index1, index2] with index1 < index2
• Requirement:
  • exactly one solution
  • cannot use the same element twice

Edge Case

• Minimum length is 2
• Negative and duplicates are allowed

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Method	Approach	Time Complexity	Space Complexity
Method 1 ⭐	Two Pointers	$O(n)$	$O(1)$

Method 1

Approach

• Use two pointers: left = 0, right = len(numbers) - 1
• While left < right:
  • Compute total = numbers[left] + numbers[right]
  • If total == target: return [left + 1, right + 1] (convert to 1-indexed)
  • If total < target: move left rightward to increase the sum
  • If total > target: move right leftward to decrease the sum

Complexity

• Time Complexity: $O(n)$
• Space Complexity: $O(1)$

Code

class Solution:
    def twoSum(self, numbers: List[int], target: int) -> List[int]:
        left = 0
        right = len(numbers) - 1
 
        while left <= right:
            total = numbers[left] + numbers[right]
            if total == target:
                return [left + 1, right + 1]
            elif total < target:
                left += 1
            else:
                right -= 1

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Common Mistake

• Forgetting the output is 1-indexed (+1)