[leetcode]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.

分析：

给定数组元素排好序，要求返回和为目标值的两个元素的下标，下标从1开始。可以用双指针法，left指针从头遍历，right指针从尾遍历，判断两数之后是否为目标值，若是，将下标值加1放入返回数组；若比目标值小，left+1，否则right-1，直到两指针相遇。

class Solution {
public:
    vector<int> twoSum(vector<int>& numbers, int target) {
        vector<int> res;//双指针法
        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; 
            
    }
};