Leetcode 167. Two Sum II - Input array is sorted (https://interviewrecipes.com/two-sum-ii) (https://youtu.be/i2y6LTIz_WU)

Leetcode_167.cpp

// Video Explanation: https://youtu.be/i2y6LTIz_WU
class Solution {
public:
    vector<int> twoSum(vector<int>& numbers, int target) { 
        int n = numbers.size(); 
        int left = 0;     // Smallest number.
        int right = n-1;  // Largest number.
        while (numbers[left] + numbers[right] != target) {
            if (numbers[left] + numbers[right] < target) { // Current sum is not sufficient
                                                           // to reach the target.
                left++;   // Increase the smaller number.
            } else {      // Current sum exceeds the target.
                right--;  // Decrease the larger number.
            }
        }
        return {left+1, right+1}; // Yay, solved.
    } 
};

// Time Complexity: O(n).
// Space Complexity: O(1).