167. Two Sum II - Input array is sorted@python

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.

原题地址: Two Sum II - Input array is sorted

难度: Easy

思路1:

采用缓存的方式,用字典记录每一个值的索引

代码:

class Solution(object):
    def twoSum(self, numbers, target):
        """
        :type numbers: List[int]
        :type target: int
        :rtype: List[int]
        """
        d = {}
        for idx, num in enumerate(numbers):
            if target - num in d:
                idx1 = d[target-num]
                return [idx1+1, idx+1]
            else:
                d[num] = idx

时间复杂度: O(n)

空间复杂度: O(n)

思路2:

因为数组已经排序了,可以分别在数组首尾放置一指针,逐渐向中间夹逼

代码:

class Solution(object):
    def twoSum(self, numbers, target):
        """
        :type numbers: List[int]
        :type target: int
        :rtype: List[int]
        """
        i, j = 0, len(numbers)-1
        while i < j:
            val = numbers[i] + numbers[j]
            if val == target:
                return [i+1, j+1]
            elif val > target:
                j -= 1
            else:
                i += 1

时间复杂度: O(n)

空间复杂度: O(1)