Binary Search Tree: In-Order Predecessor And Successor
February 12, 2013
We have seen several flavors of binary search trees, including classic bsts, treaps, red/black trees, and avl trees. For all of them we provided a lookup
function that returned a requested key/value pair. In today’s exercise we write functions that return the predecessor and successor of a requested key, allowing a program to walk the tree in order. There are two common variants of these functions, one using parent pointers and one without; our exercise will use the variant without parent pointers, which is generally more useful.
We give the basic logic for the successor
function; the predecessor
function is similar, but reversed. A recursive function descends the tree searching for the requested key, keeping track of the current tree as the possible successor every time if follows the left sub-tree. If it finds the key, its successor is the minimum element of the right sub-tree, found by chasing left sub-trees until reaching a null node. If the search reaches a null node and hence fails to find the key, the next largest key in the tree is the successor, found in the possible successor that was saved at each recursive call.
Your task is to write predecessor
and successor
functions for binary search trees. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.
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Here’s a version in Racket: Predecessor and successor in a binary search tree
I went a slightly different direction than the solution here with a shared function to return the node containing the value we were searching for and the last left and right branch all at once (the ability to return multiple
value
s is always nice). That made the actualpredecessor
andsuccessor
functions somewhat simpler:I also included code for using this to implement a sorting algorithm. Dump everything into a tree, find the
minimum
, then repeatedly callsuccessor
until you run out of elements. Am I incorrect in thinking that this is actually stillO(n log n)
, albeit with a higher constant than most? It sorts a million element list only about 3x slower than a naive implementation ofquicksort
did, but insertion sort (knownO(n^2)
) didn’t even finish before I got bored.I finally got around to this challenge. Here’s the code for successor in python.