167. Two Sum II - Input array is sorted [medium] (Python)

题目链接

https://leetcode.com/problems/two-sum-ii-input-array-is-sorted/

题目原文

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.

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

思路方法

注意题目说了两个重要条件：1，有序数组；2，有唯一解。所以解的两个数一定都是数组中唯一存在的数。

思路一

利用两个指针从数组的两侧开始向中间移动，寻找第一对和为target的两个数即为所求。

代码

class Solution(object):
    def twoSum(self, numbers, target):
        """
        :type numbers: List[int]
        :type target: int
        :rtype: List[int]
        """
        left, right = 0, len(numbers) - 1
        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

思路二

扫描数组，用字典记录扫描历史，并判断可能成对的另一个数是否在数组中。

代码

class Solution(object):
    def twoSum(self, numbers, target):
        """
        :type numbers: List[int]
        :type target: int
        :rtype: List[int]
        """
        num_dict = {}
        for i, num in enumerate(numbers):
            if (target - num) in num_dict:
                return [num_dict[target - num], i + 1]
            num_dict[num] = i + 1