Sum of Distances in Tree.java

class Solution {
    private Map<Integer, List<Integer>> getGraph(int[][] edges) {
        Map<Integer, List<Integer>> graph = new HashMap<>();
        for(int[] edge: edges) {
            graph.putIfAbsent(edge[0], new LinkedList<>());
            graph.putIfAbsent(edge[1], new LinkedList<>());
            
            graph.get(edge[0]).add(edge[1]);
            graph.get(edge[1]).add(edge[0]);       
        }
        
        return graph;
    }
    
    public int[] sumOfDistancesInTree(int n, int[][] edges) {     
        if(n < 2 || edges == null) {
            return new int[]{0};
        }
        
        int[] countSubNodes = new int[n];
        Arrays.fill(countSubNodes, 1);
        int[] distances = new int[n];
        Map<Integer, List<Integer>> graph = getGraph(edges);
        postOrderTraversal(0, -1, countSubNodes, distances, graph);
        preOrderTraversal(0, -1, countSubNodes, distances, graph, n);
        return distances;
    }
    
    private void postOrderTraversal(int node, int parent, int[] countSubNodes, int[] distances, Map<Integer, List<Integer>> graph) {
        List<Integer> children = graph.get(node);
        for(int child: children) {
            if(child != parent) {
                postOrderTraversal(child, node, countSubNodes, distances, graph);
                countSubNodes[node] += countSubNodes[child];
                distances[node] += distances[child] + countSubNodes[child];
            }
        }
    }
    
    private void preOrderTraversal(int node, int parent, int[] countSubNodes, int[] distances, Map<Integer, List<Integer>> graph, int n) {
        List<Integer> children = graph.get(node);
        for(int child: children) {
            if(child != parent) {
				distances[child] = distances[node] + (n - countSubNodes[child]) - countSubNodes[child];
				preOrderTraversal(child, node, countSubNodes, distances, graph, n);            
            }
        }
    }
}``