1. 两数之和 - 力扣（LeetCode）

第一次提交

 1 class Solution(object):
 2     def twoSum(self, nums, target):
 3         """
 4         :type nums: List[int]
 5         :type target: int
 6         :rtype: List[int]
 7         """
 8         length = len(nums)
 9         for i in range(length-1):
10             for j in range(i+1, length):
11                 if nums[i] + nums[j] == target:
12                     self = [i,j]
13                     return self

执行用时1990ms 时间复杂度还是O(n2)

因为这是用的数组的方式求解，后面用哈希求解时时间复杂度会降到O(1)

方法二：哈希

Python中用dict来实现哈希

用下标i遍历数组，每个都看看哈希表里有没有满足target - nums[i]的key，有的话return答案，没有的话就存入i及对应值

 1 class Solution(object):
 2     def twoSum(self, nums, target):
 3         """
 4         :type nums: List[int]
 5         :type target: int
 6         :rtype: List[int]
 7         """
 8         length = len(nums)
 9         help_dict = {}
10         i = 0
11         while i < length:
12             if target - nums[i] in help_dict:
13                 return [i, help_dict[target - nums[i]]]
14             else:
15                 help_dict[nums[i]] = i
16             i += 1
17         return []
18 
19