1. 两数之和

1.暴力枚举

class Solution {
    func twoSum(_ nums: [Int], _ target: Int) -> [Int] {
        var result = [Int]()
        if nums.count < 2 {
            return result
        }
        var p_1 = 0
        while p_1 < nums.count {
            for index in (p_1+1)..<nums.count {
                if nums[p_1] + nums[index] == target {
                    result.append(p_1)
                    result.append(index)
                    return result
                }
            }
            p_1+=1
        }
        return result
    }
}

2.使用哈希表降低时间复杂度到O(n)

class Solution {
    func twoSum(_ nums: [Int], _ target: Int) -> [Int] {
        var hashTable: [Int: Int] = [:]
        for index in 0..<nums.count {
            if hashTable[target - nums[index]] != nil {
                return [hashTable[target - nums[index]]!, index]
            }
            hashTable[target - nums[index]] = index
        }
        return []
    }
}