main.clj

(ns shpn.core
  (:gen-class)
  (:require [clojure.java.io :as io]
            [clojure.string :as s]))

(def input "nodes.txt")
(def findA "Node3")
(def findB "Node4")

(defn read-source
  "Read source and build hashmap where keys is child and value is parent.
  It's possible to have nil value that means not connected node"
  [fname]
  (let [rows (-> fname io/resource slurp (s/split #"\n"))
        pairs (map #(s/split % #" ") rows)]
    (zipmap (map first pairs) (map second pairs))))

(defn path-for
  "Get path to root node."
  [tree node]
  (loop [acc [node] node node]
    (let [parent (get tree node)]
      (if-not parent
        acc
        (recur (conj acc parent) parent)))))

(defn -main
  "Find lca for given tree and nodes."
  [& args]
  (let [tree (read-source input)
        pathA (path-for tree findA)
        pathB (path-for tree findB)]
    (first (filter (set pathA) pathB))))

;; (-main)
;; Result: "Node1"