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.

分析：

給定陣列元素排好序，要求返回和為目標值的兩個元素的下標，下標從1開始。可以用雙指標法，left指標從頭遍歷，right指標從尾遍歷，判斷兩數之後是否為目標值，若是，將下標值加1放入返回陣列；若比目標值小，left+1，否則right-1，直到兩指標相遇。

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