LeetCode #167 Two Sum II - Input array is sorted

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Problem

Given an array of integers that is already sorted in ascending order, find two numbers such that they add up to a specific target number.

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2.

Example

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Input: numbers = [2,7,11,15], target = 9
Output: [1,2]
Explanation: The sum of 2 and 7 is 9. Therefore index1 = 1, index2 = 2.

Note

  • Your returned answers (both index1 and index2) are not zero-based.
  • You may assume that each input would have exactly one solution and you may not use the sameelement twice.

Solution

Method: Two Pointers

Time Complexity: O(n)

Space Complexity: O(1)

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

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