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.

Note:

• 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: [1,2]
Explanation: The sum of 2 and 7 is 9. Therefore index1 = 1, index2 = 2.

vector<int> twoSum(vector<int>& numbers, int target) { // time: O(n); space: O(1)
    int i = 1, j = numbers.size(); // non-zero based
    while (i < j) {
        int sum = numbers[i - 1] + numbers[j - 1];
        if (sum == target) return {i, j};
        else if (sum < target) ++i;
        else --j;
    }
    return vector<int>();
}