LeetCode——Two Sum

LeetCode刷题之Two Sum

• 题目简介
  Given an array of integers, return indices of the two numbers such that they add up to a specific target.

• 示例

  Given nums = [2, 7, 11, 15], target = 9,
  
  Because nums[0] + nums[1] = 2 + 7 = 9,
  return [0, 1].

• 解法

  1. 暴力解法，两层循环嵌套，时间复杂度 O(n^2), 空间复杂度 O(1)

     class Solution:
         def twoSum(self, nums: List[int], target: int) -> List[int]:
             for i in range(len(nums):
                 for j in range(i+1, len(nums)):
                     if nums[i] + nums[j] == target:
                         return i, j 

  2. 使用字典，字典的key为数值，value为数组的索引，在每次循环遍历时，先求出另一个数的值，判断字典的key是否包含该值，如果存在，就返回字典对应值得value和当前循环的索引，如果不存在，将当前值和索引添加到字典中去。 时间复杂度 O(n)， 空间复杂度 O(n)

     class Solution:
     def twoSum(self, nums: List[int], target: int) -> List[int]:
         if len(nums) == 0:
             return "None"
         else:
             dic = {}
             for i in range(len(nums)):
                 anthor_number = target - nums[i]
                 if anthor_number in dic:
                     return dic[anthor_number], i
                 dic[nums[i]] = i