261-Graph-Valid-Tree.java

public class Solution {

    private Map<Integer, List<Integer>> adjacencyList = new HashMap<>();

    public boolean validTree(int n, int[][] edges) {
        if (n == 0 || n == 1) return true;

        if (edges.length == 0) return false;

        for (var edge : edges) {
            var node1 = edge[0];
            var node2 = edge[1];
            adjacencyList.putIfAbsent(node1, new ArrayList<>());
            adjacencyList.putIfAbsent(node2, new ArrayList<>());
            adjacencyList.get(node1).add(node2);
            adjacencyList.get(node2).add(node1);
        }

        Set<Integer> visited = new HashSet<>();

        return (
            depthFirstSearch(edges[0][0], -1, visited) && visited.size() == n
        );
    }

    private boolean depthFirstSearch(
        int node,
        int previous,
        Set<Integer> visited
    ) {
        if (visited.contains(node)) return false;

        visited.add(node);

        for (var neighbor : adjacencyList.get(node)) {
            if (neighbor == previous) continue;

            if (!depthFirstSearch(neighbor, node, visited)) return false;
        }

        return true;
    }
}