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, and you may not use the same element twice.

Example:

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

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

采用Hashtable或Dictionary来辅助存储，提升时间性能：

public class Solution {
    public int[] TwoSum(int[] nums, int target) {
        Hashtable numsTable = new Hashtable();
        
        for (int i = 0; i < nums.Length; ++i) {
            if (numsTable.Contains(target - nums[i])) {
                //使用Hashtable需要将其value转化为int，而使用Dictionary不需要，因为Hashtable中存储的是Object
                return new int[] {(int)numsTable[target-nums[i]], i};
            } else if (!numsTable.Contains(nums[i])) {
                numsTable.Add(nums[i], i);
            }
        }
        return new int[] {};
    }
}