Binary Tree Level Order Traversal

bfs

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if(root == null)
            return result;
        ArrayList<TreeNode> parents = new ArrayList<TreeNode>();
        parents.add(root);
        ArrayList<Integer> tmp = new ArrayList<Integer>();
        tmp.add(root.val);
        result.add(tmp);
        travel(result, parents);
        return result;
    }
    
    private void travel(ArrayList<ArrayList<Integer>> result, ArrayList<TreeNode> parents)
    {
        if(parents.size() == 0)
            return;
        ArrayList<TreeNode> nodes = new ArrayList<TreeNode>();
        for(TreeNode i: parents){
            if(i.left != null)
                nodes.add(i.left);
            if(i.right != null)
                nodes.add(i.right);
        }
        parents.clear();
        parents.addAll(nodes);
        if(nodes.size() > 0)
        {
            ArrayList<Integer> tmp = new ArrayList<Integer>();
            for(TreeNode i: nodes){
                tmp.add(i.val);
            }
            result.add(tmp);
        }
        travel(result, parents);
    }
}