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)

class Solution {
    func twoSum(_ nums: [Int], _ target: Int) -> [Int] {
        var indexForNum : Int?
        var indexForAnotherNum : Int?
        
        for (index, value) in nums.enumerated() {
            if nums.contains(target - value) {
                
                indexForNum = index
                indexForAnotherNum = nums.firstIndex(of: target - value)!
                
                if indexForAnotherNum == indexForNum {
                    continue
                }
                break
            }
        }
        return [indexForNum!,indexForAnotherNum!]
    }
}

 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]
    }
}