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].

🐹翻译:
Two Sum：给一个数组，每一位是一个整数。再给一个整数作为“目标”。数组中会有两个数的和，恰好为这个“目标”。找到这两个数，并返回它们的下标。可以假定给出的输入只有一个解，而且一个数不能用两次。例子：数组 nums 是 [2, 7, 11, 15]，目标是 9。因为 nums[0] + nums[1] = 2 + 7 = 9，所以返回 [0, 1]

class Solution {
    func twoSum(_ nums: [Int], _ target: Int) -> [Int] {
        var mapper = [Int : Int]()
        for idx in 0..<nums.count {
            if let other = mapper[target - nums[idx]]{
                return [other,idx]
            }else{
                mapper[nums[idx]] = idx
            }
        }
        return []
    }
}