Two Sum II – Input Array Is Sorted LeetCode Solution | Easy Approach

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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 index2added 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 * 104
  • -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.

Two Sum II – Input Array Is Sorted Solutions

Time: O(n)
Space: O(1)

C++

 class Solution {
 public:
  vector<int> twoSum(vector<int>& numbers, int target) {
    int l = 0;
    int r = numbers.size() - 1;

    while (numbers[l] + numbers[r] != target)
      if (numbers[l] + numbers[r] < target)
        ++l;
      else
        --r;

    return {l + 1, r + 1};
  }
};

Java

 
class Solution {
  public int[] twoSum(int[] numbers, int target) {
    int l = 0;
    int r = numbers.length - 1;

    while (numbers[l] + numbers[r] != target)
      if (numbers[l] + numbers[r] < target)
        ++l;
      else
        --r;

    return new int[] {l + 1, r + 1};
  }
}

Python

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

    while l < r:
      sum = numbers[l] + numbers[r]
      if sum == target:
        return [l + 1, r + 1]
      if sum < target:
        l += 1
      else:
        r -= 1

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