# 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.

# Solution

Approach 1: Two pointers.

# Code (Python)

Approach 1:

# Code (C++)

Approach 1:

class Solution {
public:
    vector<int> twoSum(vector<int>& numbers, int target) {
        vector<int> res;
        int head = 0;
        int tail = numbers.size() - 1;
        while (head < tail)
        {
            int sum = numbers[head] + numbers[tail];
            if (sum == target)
            {
                res.push_back(head+1);
                res.push_back(tail+1);
                break;
            }
            else if (sum < target)
                head++;
            else
                tail--;
        }
        return res;
    }
};