---
title: '167. Two Sum II - Input Array Is Sorted'
description: 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
icon: dot
sidebar:
label: 'Two Sum II - Input Array Is Sorted'
badge: 'Medium'
---
Array
::::warning
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.
## Approach
```mermaid
flowchart TD
S(["twoSum(numbers, target)"]) --> I["i = 0, j = len - 1"]
I --> W{"i < j?"}
W -- no --> E(["return []"])
W -- yes --> C["c = numbers[i] + numbers[j]"]
C --> Q{"c vs target"}
Q -- equal --> R(["return [i+1, j+1] — 1-indexed"])
Q -- "c < target" --> L["i += 1 — grow the sum"]
L --> W
Q -- "c > target" --> H["j -= 1 — shrink the sum"]
H --> W
```
## Solution
```py
class Solution:
def twoSum(self, numbers: List[int], target: int) -> List[int]:
i = 0
j = len(numbers) - 1
while i < j:
c = numbers[i] + numbers[j]
if c == target:
return [i + 1, j + 1]
elif c < target:
i+=1
else:
j-=1
return []
```
## Explanation