adding lca struct for O(log(n)) lca and dist queries

[ThomaFM]

struct based lca, includes dist function since it is commonly used. particularly long (62 character) initialization line might need to be refined later.

Summary

Includes lca and dist function. Initializes LCA structure allowing for lowest common ancestor queries given a (undirected) zero based adjacency list representing a tree. Tree is rooted at 0 by default (perhaps a parameter may be desirable?)

Submission Link/Screenshots

https://cses.fi/problemset/result/10795064/

Checklist

• [x] AC on at least 1 problem
• [x] Using tabs for indentation
• [x] Longest line at most 63 characters (with tab length 2)
• [x] Well documented

[azurerazor]

also forgot to mention that the filename should probably be all caps (LCA instead of lca)