1. Two Sum

leetcode1.cs

using System.Collections;

public class Solution
{
    public int[] TwoSum(int[] nums, int target)
    {
        int pair;
        Hashtable ht = new Hashtable();
        for (int i = 0; i < nums.Length; i++)
        {
            if (ht.ContainsKey(nums[i]))
            {
                ht[nums[i]] = i;
            }
            else
            {
                ht.Add(nums[i], i);
            }
        }
        for (int i = 0; i < nums.Length; i++)
        {
            pair = target - nums[i];
            if (ht.ContainsKey(pair))
            {
                if ((int)ht[pair] != i)
                {
                    return new int[] { i, (int)ht[pair] };
                }
            }
        }
        return new int[] { 0, 0 };
    }
}