Lowest common ancestor P49809

Statement

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The lowest common ancestor (LCA) of two nodes x and y in a tree is the lowest (i.e. deepest) node that has both x and y as descendants, where we define each node to be a descendant of itself.

For instance, in the following tree, 5 is the LCA of 1 and 9, and 6 is the LCA of 1 and 0:

Write a function Tree lowest_common_ancestor (Tree t, int x, int y); that returns the node that corresponds to the LCA of x and y in a binary tree of integers. You can assume that t contains both x and y and that t does not contain repeated elements.

Most of the program is already writen for you. Download it! It reads several trees in preorder with leaves marked with −1 and, for each of these, reads severals pairs of values and prints their LCA. You just have to specify and implement the lowest_common_ancestor() function (and other helper functions, should you need them). Also, write a comment with the time efficiency of your algorithm.

Public test cases

Input

2

5 6 1 -1 -1 7 2 -1 -1 0 -1 -1 3 9 -1 -1 4 -1 -1

    1 9
    1 0
    6 3
    3 6
    5 5
    3 3
    5 0
    -1 -1

5 2 3 -1 -1 8 -1 -1 -1

    3 8
    3 2
    3 5
    2 5
    8 5
    -1 -1

Output

Information

Author: Jordi Petit
Language: English
Official solutions: C++
User solutions: C++ Python