560. Subarray Sum Equals K

    func subarraySum(_ nums: [Int], _ k: Int) -> Int {
        var map = [Int: Int]() //sum : count
        map[0] = 1
        var ans = 0
        var sum = 0
        for (i , num) in nums.enumerated(){
            sum += num
            if let count = map[sum - k]{
                ans += count
            }
            if let count = map[sum]{
                 map[sum] = count + 1
            }else{
                map[sum] = 1
            }
        }
        return ans
    }