leetcode 167--- Two Sum II - Input array is sorted

难度：easy

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.

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

思路：此题题意表明：list已经从小到大排列好，只会有一对数值满足要求。

           用双指针的方法，一个从头向尾端走，一个从尾部向头走，如果两个指针对应的数值相加等于9就好，如果大于9，尾部的指针走一步，如果小于9，头部的指针走一步，慢慢调整直到两数相加为target.