Alien Dictionary

There is a new alien language which uses the latin alphabet. However, the order among letters are unknown to you. You receive a list of non-empty words from the dictionary, where words are sorted lexicographically by the rules of this new language. Derive the order of letters in this language.

Example 1: Given the following words in dictionary,

[ "wrt", "wrf", "er", "ett", "rftt" ]

The correct order is: "wertf".

Example 2: Given the following words in dictionary,

[ "z", "x" ]

The correct order is: "zx".

Example 3: Given the following words in dictionary,

[ "z", "x", "z" ]

The order is invalid, so return "".

Note:

You may assume all letters are in lowercase. You may assume that if a is a prefix of b, then a must appear before b in the given dictionary. If the order is invalid, return an empty string. There may be multiple valid order of letters, return any one of them is fine.

Solution

public class Solution {
     public String alienOrder(String[] words) {
      if (words == null || words.length < 1) return "";
        if (words.length ==1) return words[0];

        Map<Character, Node> map = new HashMap<>();
        for (int i = 0; i < words.length; i++) {
            String s1 = words[i];

            for (int j = 0; j < s1.length(); j++) {
                if (!map.containsKey(s1.charAt(j))) {
                    map.put(s1.charAt(j), new Node(s1.charAt(j)));
                }
            }
        }

        for (int i = 0; i <= words.length - 2; i++) {
            String s1 = words[i];
            String s2 = words[i + 1];

            for (int j = 0; j < Math.min(s1.length(), s2.length()); j++) {
                if (s1.charAt(j) != s2.charAt(j)) {
                    if (!map.containsKey(s1.charAt(j))) {
                        map.put(s1.charAt(j), new Node(s1.charAt(j)));
                    }

                    if (!map.containsKey(s2.charAt(j))) {
                        map.put(s2.charAt(j), new Node(s2.charAt(j)));
                    }

                    Node startNode = map.get(s1.charAt(j));
                    Node endNode = map.get(s2.charAt(j));
                    DirectedEdge directedEdge = new DirectedEdge(startNode, endNode);

                    startNode.addOutEdge(directedEdge);
                    endNode.addInEdge(directedEdge);
                    break;
                }
            }
        }

        StringBuffer sb = new StringBuffer();
        int[] cyle = new int[1];
        Map<Node, Boolean> onStack = new HashMap<>();
        Map<Node, Boolean> onVisit = new HashMap<>();

        for (Node node : map.values()) {
            if (!onVisit.containsKey(node)) {
                dfs(node, onStack, onVisit, sb, cyle);
            }
        }

        if (cyle[0] == 1) {
            return "";
        } else {
            return sb.reverse().toString();
        }
    }

    private void dfs(Node currentNode, Map<Node, Boolean> onStack, Map<Node, Boolean> onVisit, StringBuffer sb, int[] cycle) {
        if (cycle[0] == 1) {
            return;
        }

        onStack.put(currentNode, true);
        onVisit.put(currentNode, true);

        for (DirectedEdge edge : currentNode.outEdges) {
            Node nextNode = edge.end;
            if (!onVisit.containsKey(nextNode)) {
                dfs(nextNode, onStack, onVisit, sb, cycle);
            } else if (onStack.containsKey(nextNode) && onStack.get(nextNode)) {
                cycle[0] = 1;
                return;
            }
        }

        sb.append(currentNode.character);
        onStack.put(currentNode, false);
    }


    private class Node {
        Character character;
        List<DirectedEdge> inEdges;
        List<DirectedEdge> outEdges;

        public Node(Character character) {
            this.character = character;
            this.inEdges = new ArrayList<>();
            this.outEdges = new ArrayList<>();
        }

        public void addInEdge(DirectedEdge edge) {
            for (int i = 0; i < inEdges.size(); i++) {
                DirectedEdge current = inEdges.get(i);
                if (current.start.character == edge.start.character && current.end.character == edge.end.character) {
                    return;
                }
            }

            inEdges.add(edge);
        }

        public void addOutEdge(DirectedEdge edge) {
            for (int i = 0; i < outEdges.size(); i++) {
                DirectedEdge current = outEdges.get(i);
                if (current.start.character == edge.start.character && current.end.character == edge.end.character) {
                    return;
                }
            }

            outEdges.add(edge);
        }
    }

    private class DirectedEdge {
        Node start;
        Node end;

        public DirectedEdge(Node start, Node end) {
            this.start = start;
            this.end = end;
        }
    }
}