Leetcode 1 Two Sum

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution.

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

Solution:

经典的利用hashtable 存已经存在的值和找match的值的问题.Runtime is O(n)

public class Solution {
    public int[] TwoSum(int[] nums, int target) {
        Hashtable hashtable = new Hashtable();
        if(nums.Count() == 0) return null;
        int diff = 0;
        int[] a = new int[2];
        for(int i =0; i< nums.Count(); i++)
        {
            diff = target - nums[i];
            if(hashtable.ContainsKey(nums[i]))
            {
                a[0] = (int)hashtable[nums[i]];
                a[1] = i;
                return a;
            }
            
        hashtable.Add(diff,i);
                   }
        return null;
        
    }
}