167. Two Sum II - Input array is sorted

• Difficulty: Easy

• Topics: Array, Two Pointers, Binary Search

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Problem:

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.

Solutions:

class Solution {
public:
    vector<int> twoSum(vector<int>& numbers, int target) {
        int left = 0;
        int right = numbers.size() - 1;
        while (left < right) {
            if (numbers[left] + numbers[right] == target)   return {left + 1, right + 1};
            else if (numbers[left] + numbers[right] < target) {
                ++left;
            } else {
                --right;
            }
        }

        return {0, 0};
    }
};