167. Two Sum II - Input Array Is Sorted

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 <= nums.length <= 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.

Answer

func twoSum(numbers []int, target int) []int {
    l,r := 0, len(numbers) - 1
    var t int
    for l < r { 
        t = numbers[l] + numbers[r]
        if t == target {
            return []int{l + 1, r + 1}
        }

        if t > target{
            r -=1
        } else{
            l += 1
        }
    }
    return nil
}