Python中关于局部变量范围的问题

Question

I am solving an algorithm problem in https://leetcode.com/problems/two-sum .

Here is my code:

class Solution:
    def twoSum(self, nums, target):
        list_length = len(nums)
        result = list()
        for i in range(0, list_length):
            for j in range(i+1, list_length):
                s = i + j
                if s == target:
                    result.append(i)
                    result.append(j)
                    return result

s = Solution()
ts = s.twoSum([2, 7, 11, 15], 9)
print(ts)

The output is None .

I want to know why is it None rather than [0,1] .

Answer 1

When calculating s you are summing the indeces ( i and j ) you are iterating through rather than the elements at those indeces. This works:

class Solution:
    def twoSum(self, nums, target):
        list_length = len(nums)
        result = list()
        for i in range(list_length):
            for j in range(1, list_length):
                s = nums[i] + nums[j]
                if s == target:
                    result.append(i)
                    result.append(j)
                    return result
s = Solution()
ts = s.twoSum([2, 7, 11, 15], 27)
print(ts)

Answer 2

Indentation is important, and both i and j references are to the index within the list of nums example, [0,3,4] so num[0] is 0 etc. etc. And it's a good habit to use a main method at the bottom although not necessary depending on what you are doing. Hope this helps..

class Solution:

   def twoSum(self, nums, target):
      list_length = len(nums)
      # actually use the list() functionality here not 'list()'but '[]'
      result = []
      for i in range(list_length):
         for j in range(1, list_length):
            s = nums[i] + nums[j]
            if s == target:
              result.append(i)
              result.append(j)
              return result


if __name__ == "__main__":
  #use a main method if possible
  s = Solution()
  ts = s.twoSum([2, 7, 11, 15], 9)
  print(ts)

Answer 3

I personally find a couple things that your code could do better. First there's nothing in your class as it isn't indented. Second I find the append method to be tedious. You can initialize a list a with a = [""]* length where this will a create a list a with a specified length.