# 问题 原题链接：[https://leetcode.com/problems/two-sum/](https://leetcode.com/problems/two-sum/) 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]. ``` # 分析过程 - 输入：nums = [2, 7, 11, 15], target = 9 - 输出：[0, 1] - 思路：遍历数组，用数组的元素做 key，索引做 value 存入字典。遍历时寻找字典里有没有以 target - key 的元素，如果有的话取出下标返回即可。 # 解决方法 ```swift class Solution { func twoSum(_ nums: [Int], _ target: Int) -> [Int] { var dict = [Int: Int]() for (i, num) in nums.enumerated() { if let j = dict[target - num] { return [j, i] } dict[num] = i } fatalError("No valid outputs") } } ```