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LeetCode | 0167. Two Sum II-Input array is sorted两数之和II【Python】

题目

英文题目链接

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.

翻译

中文题目链接

给定一个已按照升序排列 的有序数组,找到两个数使得它们相加之和等于目标数。

函数应该返回这两个下标值 index1 和 index2,其中 index1 必须小于 index2。

说明:

  • 返回的下标值(index1 和 index2)不是从零开始的。
  • 你可以假设每个输入只对应唯一的答案,而且你不可以重复使用相同的元素。

示例:

输入: numbers = [2, 7, 11, 15], target = 9
输出: [1,2]
解释: 2  7 之和等于目标数 9 。因此 index1 = 1, index2 = 2 

思路

双指针

left 指针从头指向尾,right 指针从尾指向头,然后判断两数之和是否等于 target。

时间复杂度:O(n)

Python代码

class Solution(object):
    def twoSum(self, numbers, target):
        """
        :type numbers: List[int]
        :type target: int
        :rtype: List[int]
        """
        left = 0  # 从头指向尾
        right = len(numbers) - 1  # 从尾指向头
        while left < right:
            if numbers[left] + numbers[right] == target:
                return [left + 1, right + 1]
            elif numbers[left] + numbers[right] > target:
                right -= 1 
            else:
                left += 1 
        return []

 

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