Cool - Construct a graph. Each node has a word of four letters. Adjacent nodes are all words with one letter different from this node. So, HEAD node has neighbors, READ, BEAD, DEAD, etc. DEAD's neighbors are DEED, DEAN, etc. Once the graph is constructed, problem is one of computing path from HEAD to tail.

//use a hash set for a fast check if a word is already in the dictionary static HashSet Dictionary = new HashSet(); //dictionary used to find the parent in every node in the graph and to avoid traversing an already traversed node static Dictionary parents = new Dictionary(); public static List FindPath(List input, string start, string end) { char[] allcharacters = {'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'}; foreach (string s in input) Dictionary.Add(s); List currentFrontier = new List(); List nextFrontier = new List(); currentFrontier.Add(start); while (currentFrontier.Count > 0) { foreach (string s in currentFrontier) { for (int i = 0; i ExtractPath(string start,string end) { List path = new List(); string current = end; path.Add(end); while (current != start) { current = parents[current]; path.Add(current); } path.Reverse(); return path; }