Solution to leetcode course schedule II

Solution.java

import java.util.*;

public class Solution {
  
    public static void main(String[] args) {
        int[][] deps = {{1, 0}};
        System.out.printf("%s\n", Arrays.toString(new Solution().findOrder(2, deps)));
    }
    
    private static Integer append(int i, Map<Integer, List<Integer>> map, Map<Integer, Integer> depthMap) {
        if (depthMap.containsKey(i)) {
            if (null == depthMap.get(i)) {
                return null;
            } else {
                return depthMap.get(i);
            }
        } else if (!map.containsKey(i)) {
            depthMap.put(i, 0);
            return 0;
        } else {
            int val = 0;
            depthMap.put(i, null);
            for (int child : map.get(i)) {
                Integer sub = append(child, map, depthMap);
                if (null == sub) {
                    return sub;
                }
                val = Math.max(val, 1 + sub);
            }
            depthMap.put(i, val);
            return val;
        }
    }
    
    private static Map<Integer, List<Integer>> buildDeps(int[][] prerequisites) {
        Map<Integer, List<Integer>> deps = new HashMap<Integer, List<Integer>>();
        for (int[] pair : prerequisites) {
            int child = pair[0];
            int parent = pair[1];
            if (!deps.containsKey(parent)) {
                deps.put(parent, new ArrayList<Integer>());
            }
            deps.get(parent).add(child);
        }
        return deps;
    }
    
    private static int[] convert(List<Integer> list) {
        int[] result = new int[list.size()];
        int i=0;
        for (int x : list) result[i++] = x;
        return result;
    }
    
    public int[] findOrder(int numCourses, int[][] prerequisites) {
        if (numCourses == 0) {
            return new int[0];
        }
        Map<Integer, List<Integer>> deps = buildDeps(prerequisites);
        final Map<Integer, Integer> depthMap = new HashMap<Integer, Integer>();
        for (int i=0; i<numCourses; ++i) {
            if (null == append(i, deps, depthMap)) {
                return new int[0];
            }
        }
        List<Integer> result = new ArrayList<Integer>(depthMap.keySet());
        System.out.printf("%s\n", depthMap);
        Collections.sort(result, new Comparator<Integer>(){
            @Override
            public int compare(Integer lhs, Integer rhs) {
                return - Integer.compare(depthMap.get(lhs), depthMap.get(rhs));
            }
        });
        return convert(result);
    }
}