If following sequence of keys are inserted in a B+ tree with K(=3) pointers :…
2017
If following sequence of keys are inserted in a B+ tree with K(=3) pointers :
8, 5, 1, 7, 3, 12, 9, 6
Which of the following shall be correct B+ tree ?
Attempted by 46 students.
Show answer & explanation
Key idea: For a B+ tree with K = 3 pointers, internal nodes can have up to 2 keys and leaves hold up to 2 keys. When a leaf overflows (3 keys temporarily), split it into two leaves (left gets 2 keys, right gets 1) and promote the first key of the right leaf into the parent. If an internal node overflows, split and promote the middle key to the parent.
Insert 8: single leaf [8].
Insert 5: leaf becomes [5,8] (fits).
Insert 1: leaf temporarily [1,5,8] → split into leaves [1,5] and [8]; promote 8 to become the root key.
Insert 7: goes to the left leaf [1,5] (since 7 < 8). Left leaf becomes [1,5,7] → split into [1,5] and [7]; promote 7 into the root. Root now has keys [7,8] with three children.
Insert 3: goes to leftmost area (under 7). Left child leaf [1,5] becomes [1,3,5] → split into [1,3] and [5]; promoting 5 into the parent causes the internal node to overflow (would have three keys).
Resolve the internal overflow: split the internal node and promote the middle key 7 to form a new root. Resulting structure after this step: root key [7]; left internal node with key [5] pointing to leaves [1,3] and [5]; right internal node with key [8] pointing to leaves [7] and [8].
Insert 12: goes to rightmost leaf [8] → becomes [8,12] (fits).
Insert 9: goes to leaf [8,12] → becomes [8,9,12] → split into [8,9] and [12]; promote 12 into the right internal node so it now has keys [8,12] with three children [7], [8,9], [12].
Insert 6 (final): goes under the left internal node (since 6 < 7 but ≥ 5), into leaf [5] → becomes [5,6] (fits).
Final tree:
Root key: 7
Left internal node key: 5 with leaf children [1,3] and [5,6]
Right internal node keys: 8 and 12 with leaf children [7], [8,9], [12]
Leaf level (left to right): [1,3], [5,6], [7], [8,9], [12]
Conclusion: The correct image is the one that depicts a root key of 7, left leaves [1,3] and [5,6], and right leaves [7], [8,9], [12]. The previously blank solution has been replaced by this step-by-step reasoning and the final validated tree description.