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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