LeetCode: Two Sum II - Input Array Is Sorted

Problem:

https://leetcode.com/problems/two-sum-ii-input-array-is-sorted/

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:
                right-=1
            else:
                left+=1
        return []

Key Points:

• The array is already sorted → perfect for the two-pointer technique.

• Start two pointers: left = 0 # at the start (smallest numbers) right = n - 1 # at the end (largest numbers)

• At each step:

  • total = numbers[left] + numbers[right]

  • If total == target: → Found the exact pair → return their 1-based indices.

  • If total > target: → Sum too large → move right pointer left (right -= 1) to try a smaller number.

  • If total < target: → Sum too small → move left pointer right (left += 1) to try a larger number.

• Two-pointer works because the array is sorted:

  • Moving left pointer increases the sum.

  • Moving right pointer decreases the sum.

• Guaranteed exactly one solution → loop will always find it.

• Time Complexity: O(n)

• Space Complexity: O(1)