Simple In-Memory Database

This problem requires you to implement a simple in-memory key-value database supporting four operations:

• PUT k v: Store the value v with key k. If the key already exists, update its value.
• GET k: Retrieve the value associated with key k, or output NULL if the key does not exist.
• DELETE k: Remove the key k and its associated value if it exists.
• COUNT v: Return the number of keys that have the value v.

The commands are provided via standard input and the output for each GET and COUNT command should be printed to standard output. Use the following input format:

 n
 command1
 command2
 ...
 commandn

For example, a sample execution might be:

6
PUT key1 value1
GET key1
PUT key1 value2
GET key1
COUNT value1
COUNT value2

with the output:

value1
value2
0
1

Note: When dealing with formulas or mathematical expressions in your solution, use LaTeX format, such as $E=mc^2$.

inputFormat

The first line contains an integer (n) representing the number of commands. The next (n) lines each contain a command in one of the following formats:

• PUT k v • GET k • DELETE k • COUNT v

Here, k and v are strings without spaces.

outputFormat

For each command of type GET and COUNT, output the result on a new line to stdout.## sample

6
PUT key1 value1
GET key1
PUT key1 value2
GET key1
COUNT value1
COUNT value2

value1
value2
0
1