我应该如何修改我的代码以便 twosum 函数工作？

Question

The question is in Leetcode, which is:

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target. Each input would have exactly one solution, and you may not use the same element twice.

Here is my code for this exercise.

def twoSum(nums: List[int], target: int) -> List[int]:
    for i in range(len(nums)):
        for j in range(1,len(nums)):
            if nums[i]+nums[j]==target:
                return i,j
        return i,j
twoSum([8,3,7,9,2],9)
#Result: (0, 4)

The output is not correct as 8+2=10 ( nums[0] + nums[4] ) not equal to 9.

Why does this happen?

Answer 1

You have two issues with your code:

• Your second return will just return at the end of the first iteration of the outer loop. Since i is still at 0 and j is all the way up to len(nums) - 1 , you get (0, 4) as result. Removing it should resolve this issue.
• The innner loop starts with 1 instead of 0, but results like (1, 1) would still be possible. It is therefore necessary to adapt the inner loop based on the outer loops current state.

A possible solution would look like this:

def twoSum(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
  return -1, -1

The -1, -1 will only occur if there is no solution. Since it is stated in the question that there is always exactly one solution, this should never happen.