No output from algorithm for Two Sum problem in Leetcode

Question

The code I have written that aims to solve the Two Sum problem:

def twoSum(self, nums: List[int], target: int) -> List[int]:
   
    dict = {}

    for i in range(len(nums)):
        complement = target - nums[i]
        if complement in dict:
            return [dict[complement], i]
        dict[complement] = i

I have just started practicing on LeetCode and I am experiencing issues with solving the Two Sum problem.

The problem statement:

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

You can return the answer in any order.

My reasoning is to create a dictionary and iterate through the numbers, and for each number generate a complement number that I then look for in my dictionary, if it in fact is there, then I return the index that generates that complement and the current index i. Otherwise, I insert the key with the complement.

Somehow my function does not output anything, just two empty brackets. Below there is a sample input and correct output.

Input: nums = [3,2,4], target = 6
Output: [1,2]

Answer 1

The last line is wrong. It should read dict[nums[i]] = i, because you are storing indeces for their values. Here is the entire function with a better variable name that doesn't shadow the built-in type:

def twoSum(self, nums, target):
    dct = {}
    for i in range(len(nums)):
        complement = target - nums[i]
        if complement in dct:
            return [dct[complement], i]
        dct[nums[i]] = i

Or more concise using enumerate and storing indeces for their complement values:

def twoSum(self, nums, target):
    dct = {}
    for i, num in enumerate(nums):
        if num in dct:
            return [dct[num], i]
        dct[target - num] = i

You may notice that you had a mixture of the two approaches. You looked for the complement in dct, and also wanted to store it for the current index. One of the two needs to be the current value.