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 []
}
