I’m currently trying to solve the first Leetcode problem (Two Sum) :

“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.

Example 1:

Input: nums = [2,7,11,15], target = 9

Output: [0,1]

Explanation: Because nums[0] + nums[1] == 9, we return [0, 1].

Example 2:

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

Output: [1,2]

Example 3:

Input: nums = [3,3], target = 6

Output: [0,1]”

Here’s my code:

public class Solution {
    public int[] TwoSum(int[] nums, int target) {
        Dictionary<int, int> hash = new Dictionary<int, int>();
        int n = nums.Count();

        for (int i=0; i < n; i++) {
            int complement = target - nums[i];
            if (hash.ContainsKey(complement)) {
                int[] solution = new int[]{(int)i, (int)hash[complement]};
                return solution;
            }
            else {
                hash.Add(nums[i], i);
            }
        }

        return new int[]{};
    }

This code works fine until I get a list containing the same element several times.

For example:

Inputs : nums = [2,7,11,15], target = 9

Returns : [1,0]

Inputs : nums = [1,1,1,1,1,4,1,1,1,1,1,7,1,1,1,1,1] target = 11

Returns : “Unhandled exception. System.ArgumentException: An item with the same key has already been added. Key: 1”

So the program is trying to add another ‘1’ key in my dictionary. I think it’s because even if the value is the same, the object is different, but i can’t find how to solve this.

Any help ? Thanks!