Leetcode #981: Time Based Key-Value Store

class TimeMap {
    
    /** Initialize your data structure here. */
    private var timestamps = [String: [Int]]()
    private var dict = [Int: String]()
    
    func set(_ key: String, _ value: String, _ timestamp: Int) {
        timestamps[key, default: []].append(timestamp)
        dict[timestamp] = value
    }
    
    func get(_ key: String, _ timestamp: Int) -> String {
        if let ar = timestamps[key] {
            if ar[0] > timestamp {
                return ""
            }
            var start = 0
            var end = ar.count - 1
            while start <= end {
                let mid = start + (end - start) / 2
                if ar[mid] == timestamp {
                    return dict[ar[mid]]!
                } else if ar[mid] < timestamp {
                    start = mid + 1
                } else {
                    end = mid - 1
                }
            }
            return dict[ar[end]]!
        } else {
            return ""
        }
    }
}