Two Sum II - Input array is sorted问题及解法

问题描述：

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. Please note that 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.

示例：
Input: numbers={2, 7, 11, 15}, target=9
Output: index1=1, index2=2

问题分析：

题目给出了以下几个条件很重要：

1.数组是按照升序排列的

2.数组中存在准确的一组值满足target

因而我们可以首尾相加，遍历求解：若两数之和大于target，则末尾指针减一；若小于target，则前端指针加一；直到两数之和等于target。

过程详见代码：

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