LeetCode 1. Two Sum

给定一个整数数组，返回两个数字的索引，使得它们加起来等于一个特定的目标值。

您可以假设每个输入都有一且只有一个答案，数组中每个元素只能使用一次。

Example:

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

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

　　

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
                        int[] nums = { 7, 11, 15 ,2};

                        int target = 9;

                        var numarr = TwoSum(nums, target);

        }
        // right hash: time O(n) (degenerate when collison occured) ususlly O(1) (insert and search) space: O(n) result: 496ms
        public static  int[] TwoSum(int[] nums, int target)
        {

int[] result = new int[2]; //[0,3],[3,0]
Dictionary<int, int> table = new Dictionary<int,int>(); // use empty constructor, result: 544 ms
for (int i = 0; i < nums.Length; i++)
{
int another = target - nums[i];
if (table.ContainsKey(another))
{
int anotherIndex = table[another];
result[0] = anotherIndex > i ? i : anotherIndex;
result[1] = anotherIndex < i ? i : anotherIndex;
return result;
}
else
{
if (!table.ContainsKey(nums[i]))
{
table.Add(nums[i], i);
}
}
}

throw new Exception("no answer");

        }
    }
}