creating a graph and doing a topological sort

Author: Arjiit

BFS/alienOrder.java

@@ -0,0 +1,60 @@
+class Solution {
+    public String alienOrder(String[] words) {
+        if (words == null || words.length == 0) {
+            return "";
+        }
+        HashMap<Character, HashSet<Character>> graph = new HashMap<>();
+        int[] inDegreeMap = new int[26]; //to keep a track of indegrees to a 
+        // character
+            /* according to given dictionary with specified order, traverse every pair of words,
+         * then put each pair into graph map to build the graph, and then update inDegree map
+         * for every "nextChar" (increase their inDegree by 1 every time) */
+        for (String s: words) {
+            for( char x: s.toCharArray()) {
+                graph.putIfAbsent(x, new HashSet<>());
+            }
+        }
+    for (int i=0; i< words.length - 1; i++) {
+        String first = words[i];
+        String second = words[i+1];
+        int len = Math.min(first.length(), second.length());
+        for (int j=0; j<len; j++) {
+            if (first.charAt(j) != second.charAt(j)) {
+                char f = first.charAt(j);
+                char s = second.charAt(j);
+                if (!graph.get(f).contains(s)) {
+                    graph.get(f).add(s);
+                    inDegreeMap[(int)(s - 'a')]++;
+                }
+            }
+            break;
+        }
+    }
+        // Doing a BFS but putting all starting nodes i.e ones with inDegree 0 in the queue
+        // and then exploring their neighbours, if we get indegree 0, we add to queue to 
+        // further evaluate.
+        Queue<Character> q = new LinkedList<>();
+        StringBuilder sb = new StringBuilder();
+        for (char c: graph.keySet()) {
+            if (inDegreeMap[(int)(c-'a')] == 0) {
+                q.offer(c);
+            }
+        }
+        
+        while(!q.isEmpty()) {
+            char out = q.poll();
+            sb.append(out);
+            if (graph.get(out) == null) {
+                continue;
+            }
+            System.out.println(out);
+            for (char neighbour: graph.get(out)) {
+                inDegreeMap[(int)(neighbour -'a')]--;
+                if (inDegreeMap[(int)(neighbour - 'a')] == 0) {
+                    q.offer(neighbour);
+                }
+            } 
+        }
+        return sb.length() == graph.size() ? sb.toString() : "";
+    }
+}