359.Logger Rate Limiter

Tags: Easy Design Hash Table

Links: https://leetcode.com/problems/logger-rate-limiter/

---

Design a logger system that receive stream of messages along with its timestamps, each message should be printed if and only if it is not printed in the last 10 seconds.

Given a message and a timestamp (in seconds granularity), return true if the message should be printed in the given timestamp, otherwise returns false.

It is possible that several messages arrive roughly at the same time.

Example:

Logger logger = new Logger();

// logging string "foo" at timestamp 1
logger.shouldPrintMessage(1, "foo"); returns true; 

// logging string "bar" at timestamp 2
logger.shouldPrintMessage(2,"bar"); returns true;

// logging string "foo" at timestamp 3
logger.shouldPrintMessage(3,"foo"); returns false;

// logging string "bar" at timestamp 8
logger.shouldPrintMessage(8,"bar"); returns false;

// logging string "foo" at timestamp 10
logger.shouldPrintMessage(10,"foo"); returns false;

// logging string "foo" at timestamp 11
logger.shouldPrintMessage(11,"foo"); returns true;

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如果一条Message该被打印，只可能是两种情况：

• 之前这条Message没有出现过
• 或者两次Message的时间戳的差大于等于10

于是很自然的联想到可以用一个Hash Table去记录前面出现的时间戳，大概表示成unordered_map<string, int>，一看到要用string做键，如果这个Message很长的话，即浪费时间复制，也浪费空间，所以想到可以用字符串哈希，用哈希值作为键。

class Logger {
    typedef unsigned long long ull;
    static constexpr ull base = 13331;

    unordered_map<ull, int> um;

public:
    /** Initialize your data structure here. */
    Logger() {}

    ull getHash(const string & str)
    {
        int n = str.size();
        ull res = 0;
        for (int i = 0; i < n; ++i) {
            res = res * base + (int)str[i];
        }

        return res;
    }

    /** Returns true if the message should be printed in the given timestamp, otherwise returns false.
        If this method returns false, the message will not be printed.
        The timestamp is in seconds granularity. */
    bool shouldPrintMessage(int timestamp, string message) {
        ull key = getHash(message);
        if (um.find(key) == um.end()) {
            um[key] = timestamp;
            return true;
        }

        if (timestamp - um[key] >= 10) {
            um[key] = timestamp;
            return true;
        }

        return false;
    }
};

/**
 * Your Logger object will be instantiated and called as such:
 * Logger* obj = new Logger();
 * bool param_1 = obj->shouldPrintMessage(timestamp,message);
 */