LeetCode 167. Two Sum II - Input array is sorted

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

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note that your returned answers (both index1 and index2) are not zero-based.

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

Example

Input: numbers={2, 7, 11, 15}, target=9

Output: index1=1, index2=2

Solution

Two Pointers: Search from both sides

Time Complexity: $O(n)$

// Pair the values from both side.
// Time Complexity: O(n)
//   - Search all values in the array: O(n)
vector<int> twoSum(vector<int>& numbers, int target) {
  int left = 0; // from head of the array.
  int right = numbers.size() - 1; // from tail of the array.
  while (left < right) {
    int sum = numbers[left] + numbers[right];
    if (sum == target) {
      return vector<int> {left + 1, right + 1};
    }
    sum < target ? ++left : --right;
  }
  return vector<int> {-1, -1};
}