Time Based Key-Value Store

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"   

思路：这题就是考察，bianry search，用HashMap<String, List<Node>> 把value , time存下来之后，因为是递增的，所以可以做binary search；T: O(NlogN) S: O(N) 注意，有可能搜索的范围根本没有 小于 time 的value，此时要返回null；

class TimeMap {
    private class Node {
        public String value;
        public int time;
        public Node(String value, int time) {
            this.value = value;
            this.time = time;
        }
    }
    
    private HashMap<String, List<Node>> hashmap; 
    /** Initialize your data structure here. */
    public TimeMap() {
        hashmap = new HashMap<>();
    }
    
    public void set(String key, String value, int timestamp) {
         hashmap.putIfAbsent(key, new ArrayList<Node>());
         Node node = new Node(value, timestamp);
         hashmap.get(key).add(node);
    }
    
    public String get(String key, int timestamp) {
        if(!hashmap.containsKey(key)) {
            return ""; 
        } else {
            // contains; Do binary search on list;
            List<Node> list = hashmap.get(key);
            Node node = binarySearch(list, timestamp);
            return node == null ? "" : node.value;
        }
    }
    
    private Node binarySearch(List<Node> list, int target) {
        int start = 0;
        int end = list.size() - 1;
        
        // find first element.time <= target;
        while(start + 1 < end) {
            int mid = start + (end - start) / 2;
            if(list.get(mid).time == target) {
                return list.get(mid);
            }
            if(list.get(mid).time < target) {
                start = mid;
            } else {
                // list.get(mid).time > target
                end = mid;
            }
        }   
        // 注意有可能 start，end都不小于target，也就是说不在搜索范围; return null;
        if(list.get(end).time <= target) {
            return list.get(end);
        }
        if(list.get(start).time <= target) {
            return list.get(start);
        }
        return null;
    }
}

/**
 * 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);
 */