CTCI 4.4

Given a binary tree, design an algorithm which creates a linked list of all the nodes at each depth (e.g., if you have a tree with depth D,you'll have D linked lists).

/*Use BFS to travel the tree level by level*/

import java.util.*;

public class LevelTravel {
    public List<List<TreeNode>> levelOrder(TreeNode root) {
        List<List<TreeNode>> result = new ArrayList<List<TreeNode>>();
        List<TreeNode> temp1 = new ArrayList<TreeNode>();
        List<TreeNode> temp2 = new ArrayList<TreeNode>();
        if(root != null) temp1.add(root);
        while(temp1.isEmpty() == false) {
            ArrayList<TreeNode> array = new ArrayList<TreeNode>();
            for(TreeNode node : temp1) {
                array.add(node);
                if(node.left != null) temp2.add(node.left);
                if(node.right != null) temp2.add(node.right);
            }
            result.add(array);
            temp1.clear();
            for(TreeNode node : temp2) {
                temp1.add(node);
            }
            temp2.clear();
        }
        return result;
    }
}