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].

O(n^2)解法:

  func twoSum(_ nums: [Int], _ target: Int) -> [Int] {
        
        let count:Int = nums.count
        
        for i in 0..<count - 1 {
            
            let num:Int = nums[i]
            
            for j in (i + 1)..<count {
                
                if num + nums[j] == target {
                    return [i, j]
                }
            }
        }
        
        return []
    }

O(n)解法:

   func twoSum2(_ nums: [Int], _ target: Int) -> [Int] {
        
        var dict = [Int : Int]()
        
        for (i, num) in nums.enumerated() {
            
            if let lastIndex = dict[target - num] {
                return [lastIndex, i]
            }
            
            dict[num] = i
        }
        
        return []
    }