B Plus Tree

This is a course project for ADS. The B+ tree is assumed to store key of double type, and value of String type. Order of this B plus tree is specified from input file. Operations on this tree includes:

1. initialization, which initialize the tree with specified order;
2. insertion, which does not return anything;
3. search by key, which returns all the values with this key in any order(return null if there is no such key);
4. search by key range, which returns all key-value pairs within this key range(sorted by key in ascending order);