Leetcode学习（2）—— 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. 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.

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给定一个已经按升序排序的整数数组，找到两个数字，使它们相加到一个特定的目标数。

函数twoSum应该返回两个数字的索引，使它们相加到目标，其中index1必须小于index2。 请注意，您返回的答案（index1和index2）都不是基于零的。

您可以假设每个输入都将具有一个解决方案，您可能不会使用相同的元素两次。

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

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思路：因为整个数组是升序排列的，所以用数组首元素+尾元素与目标值进行比较，

• 若是小于目标值，则将首指针后移一位；
• 若是大于目标值，则将尾指针前移一位；
• 若恰好等于目标值，则返回首尾指针

class Solution(object):
    def towSumII(self, nums, target):
        start, end = 0, len(nums) - 1       
        while start != end:
            sum = nums[start] + nums[end]
            if sum > target:
                end -= 1
            elif sum < target:
                start += 1
            else:
                return [start+1, end+1]