BFS of graph

import java.util.*;

class Solution {
    // Function to return Breadth First Search Traversal of given graph.
    public ArrayList<Integer> bfs(ArrayList<ArrayList<Integer>> adj) {
        ArrayList<Integer> traversal = new ArrayList<>();
        Queue<Integer> queue = new LinkedList<>();
        int size = adj.size();
        boolean[] visited = new boolean[size];
        queue.add(0);
        visited[0] = true;
        while (!queue.isEmpty()) {
            int pop = queue.poll();
            traversal.add(pop);

            for (int i = 0; i < adj.get(pop).size(); i++) {
                int current = adj.get(pop).get(i);

                if (!traversal.contains(current) && visited[current] != true) {
                    queue.add(current);
                    visited[current] = true;
                }
            }
        }
        return traversal;
    }
}