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topological sort.java
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topological sort.java
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public static void main(String[] args) {
int n = 4;
int m = 5;
String[] inputs = new String[m];
inputs[0] = "bc";
inputs[1] = "dc";
inputs[2] = "ad";
inputs[3] = "bd";
inputs[4] = "ba";
char[] letters = {'a', 'b', 'c', 'd'};
Map<Character, List<Character>> graph = new HashMap<Character, List<Character>>();
Map<Character, Integer> indegrees = new HashMap<Character, Integer>();
for (char c: letters) {
graph.put(c, new ArrayList<Character>());
indegrees.put(c, 0);
}
for (String s: inputs) {
char from = s.charAt(0);
char to = s.charAt(1);
graph.get(from).add(to);
indegrees.put(to, indegrees.get(to) + 1);
}
List<Character> orderLetters = populateLetterInOrder(graph, indegrees);
System.out.println("order letters:" + orderLetters);
}
private static List<Character> populateLetterInOrder(Map<Character, List<Character>> graph,
Map<Character, Integer> indegrees) {
List<Character> res = new ArrayList<Character>();
Queue<Character> queue = new LinkedList<Character>();
boolean[] visited = new boolean[256];
for (Map.Entry<Character, Integer> entry: indegrees.entrySet()) {
if (entry.getValue() == 0)
queue.add(entry.getKey());
}
while (!queue.isEmpty()) {
char c = queue.poll();
res.add(c);
visited[c] = true;
for (char neighbour: graph.get(c)) {
if (visited[neighbour])
break; // cycle detected
indegrees.put(neighbour, indegrees.get(neighbour)-1);
if (indegrees.get(neighbour) == 0)
queue.add(neighbour);
}
}
return res;
}