LeetCode 167. Two Sum II – Input array is sorted

Description

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

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

Return the indices of the two numbers (1-indexed) as an integer array answer of size 2, where 1 <= answer < answer <= numbers.length.

You may assume that each input would have exactly one solution and you may not use the same element twice.

Example 1:

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.

Example 2:

Input: numbers = [2,3,4], target = 6
Output: [1,3]

Example 3:

Input: numbers = [-1,0], target = -1
Output: [1,2]

Constraints:

• 2 <= numbers.length <= 3 * 104
• -1000 <= numbers[i] <= 1000
• numbers is sorted in increasing order.
• -1000 <= target <= 1000
• Only one valid answer exists.

Explanation

Use two pointers technique.

Python Solution

class Solution:
def twoSum(self, numbers: List[int], target: int) -> List[int]:
i = 0
j = len(numbers) - 1

while i < j:
if numbers[i] + numbers[j] < target:
i += 1
elif numbers[i] + numbers[j] > target:
j -= 1
else:
return [i + 1, j + 1]

return []
• Time Complexity: O(N).
• Space Complexity: O(1).