Twenty Percent Knowledge: topological sort

Sunday, July 17, 2016

Topological Sorting

This is one algorithm I want to remember. My first thought is to build adjacency list and then traversal it. That would be wrong because child might appears before some of its parent.

/**
 * Definition for Directed graph.
 * class DirectedGraphNode {
 *     int label;
 *     ArrayList<DirectedGraphNode> neighbors;
 *     DirectedGraphNode(int x) { label = x; neighbors = new ArrayList<DirectedGraphNode>(); }
 * };
 */
public class Solution {
    /**
     * @param graph: A list of Directed graph node
     * @return: Any topological order for the given graph.
     */    
    public ArrayList<DirectedGraphNode> topSort(ArrayList<DirectedGraphNode> graph) {
        // write your code here
        //build adjacency list and then dfs to build output
        
    ArrayList<DirectedGraphNode> result = new ArrayList<DirectedGraphNode>();
    if(graph==null||graph.size()==0)return result;
    //mark neighbors, and their referred times
        HashMap<DirectedGraphNode, Integer> map = new HashMap();
        for (DirectedGraphNode node : graph) {
            for (DirectedGraphNode neighbor : node.neighbors) {
                if (map.containsKey(neighbor)) {
                    map.put(neighbor, map.get(neighbor) + 1);
                } else {
                    map.put(neighbor, 1); 
                }
            }
        }
        //adding the start nodes the first
        Queue<DirectedGraphNode> q = new LinkedList<DirectedGraphNode>();
        for (DirectedGraphNode node : graph) {
            if (!map.containsKey(node)) {
                q.offer(node);
                result.add(node);
            }
        }
        //then adding the neighbors
        while (!q.isEmpty()) {
            DirectedGraphNode node = q.poll();
            for (DirectedGraphNode n : node.neighbors) {
                map.put(n, map.get(n) - 1);//descrease the counter
                if (map.get(n) == 0) {
                    result.add(n);//add neighbor node when its reference becomes 0
                    //all its parents have been added
                    q.offer(n);//start to process itself
                }
            }
        }
        return result;
    }
}