LeetCode

要求：

给定一个整数数组 nums 和一个目标值 target，请你在该数组中找出和为目标值的那两个整数，并返回他们的数组下标。你可以假设每种输入只会对应一个答案。但是，你不能重复利用这个数组中同样的元素。

优化前时间复杂度O(n^2)

 1 class Solution {
 2     func twoSum(_ nums: [Int], _ target: Int) -> [Int] {
 3         var indexForNum : Int?
 4         var indexForAnotherNum : Int?
 5         
 6         for (index, value) in nums.enumerated() {
 7             if nums.contains(target - value) {
 8                 
 9                 indexForNum = index
10                 indexForAnotherNum = nums.firstIndex(of: target - value)!
11                 
12                 if indexForAnotherNum == indexForNum {
13                     continue
14                 }
15                 break
16             }
17         }
18         return [indexForNum!,indexForAnotherNum!]
19     }
20 }

优化后

class Solution {
    func twoSum(_ nums: [Int], _ target: Int) -> [Int] {
        var indexForNum : Int = 0
        var indexForAnotherNum : Int = 0
        var dictionary : [Int : Int] = [:]
        
        for (index, arrValue) in nums.enumerated() {
            if dictionary.values.contains(target - arrValue) {
                indexForAnotherNum = index
                for (key,dicValue) in dictionary {
                    if dicValue == target - arrValue {
                        indexForNum = key
                        break
                    }
                }
                break
            }
            dictionary.updateValue(arrValue, forKey: index)
        }
        return [indexForNum,indexForAnotherNum]
    }
}