LeetCode——Two Sum

LeetCode刷題之Two Sum

• 題目簡介
  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，兩層循環嵌套，時間複雜度 O(n^2), 空間複雜度 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爲數值，value爲數組的索引，在每次循環遍歷時，先求出另外一個數的值，判斷字典的key是否包含該值，若是存在，就返回字典對應值得value和當前循環的索引，若是不存在，將當前值和索引添加到字典中去。 時間複雜度 O(n)， 空間複雜度 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