反向迭代器reverse_iterator

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本文介绍了一种时间基键值存储的数据结构实现，包括两个主要操作：设置和获取。通过实例展示了如何存储键值对并根据时间戳检索最近的值。

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981. Time Based Key-Value Store

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Create a timebased key-value store class TimeMap, that supports two operations.

1. set(string key, string value, int timestamp)

Stores the key and value, along with the given timestamp.

2. get(string key, int timestamp)

Returns a value such that set(key, value, timestamp_prev) was called previously, with timestamp_prev <= timestamp.
If there are multiple such values, it returns the one with the largest timestamp_prev.
If there are no values, it returns the empty string ("").

Example 1:

Input: inputs = ["TimeMap","set","get","get","set","get","get"], inputs = [[],["foo","bar",1],["foo",1],["foo",3],["foo","bar2",4],["foo",4],["foo",5]]
Output: [null,null,"bar","bar",null,"bar2","bar2"]
Explanation:   
TimeMap kv;   
kv.set("foo", "bar", 1); // store the key "foo" and value "bar" along with timestamp = 1   
kv.get("foo", 1);  // output "bar"   
kv.get("foo", 3); // output "bar" since there is no value corresponding to foo at timestamp 3 and timestamp 2, then the only value is at timestamp 1 ie "bar"   
kv.set("foo", "bar2", 4);   
kv.get("foo", 4); // output "bar2"   
kv.get("foo", 5); //output "bar2"

Example 2:

Input: inputs = ["TimeMap","set","set","get","get","get","get","get"], inputs = [[],["love","high",10],["love","low",20],["love",5],["love",10],["love",15],["love",20],["love",25]]
Output: [null,null,null,"","high","high","low","low"]

class TimeMap {
public:
    /** Initialize your data structure here. */
    TimeMap() {
        
    }
    
    void set(string key, string value, int timestamp) {
        Map[key][timestamp] = value;
    }
    
    string get(string key, int timestamp) {
        string Result = "";
        if(Map.count(key) == 0){
            return Result;
        }
        if(Map[key].count(timestamp) != 0){
            Result = Map[key][timestamp];
            return Result;
        }
        map<int,string>::reverse_iterator it = Map[key].rbegin();
        for(it;it != Map[key].rend();it++){
            if(it->first > timestamp){
                continue;
            }
            Result = it->second;
            return Result;
        }
        return Result;
    }
    private:
        unordered_map<string,map<int,string>> Map;
};

/**
 * Your TimeMap object will be instantiated and called as such:
 * TimeMap* obj = new TimeMap();
 * obj->set(key,value,timestamp);
 * string param_2 = obj->get(key,timestamp);
 */