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Two Sum II - Input Array Is Sorted

Easy
Two Pointers

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.

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. We return [1, 2].

Example 2

Input: numbers = [2,3,4], target = 6
Output: [1,3]
Explanation: The sum of 2 and 4 is 6. Therefore index1 = 1, index2 = 3. We return [1, 3].

Example 3

Input: numbers = [-1,0], target = -1
Output: [1,2]
Explanation: The sum of -1 and 0 is -1. Therefore index1 = 1, index2 = 2. We return [1, 2].

Constraints

  • •2 <= numbers.length <= 3 * 10^4
  • •-1000 <= numbers[i] <= 1000
  • •numbers is sorted in non-decreasing order
  • •-1000 <= target <= 1000
  • •The tests are generated such that there is exactly one solution.
Show Hints (3)
Hint 1: Since the array is sorted, what technique can efficiently find two numbers that sum to a target?
Hint 2: Start with pointers at both ends. If the sum is too large, move the right pointer left. If too small, move the left pointer right.
Hint 3: This is the classic two pointers technique on a sorted array with O(n) time and O(1) space.
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