Binary Tree Level Order Traversal

Problem

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

Example

Given binary tree {3,9,20,#,#,15,7},

  3
 / \
9  20
  /  \
 15   7

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

Challenge

Challenge 1: Using only 1 queue to implement it.

Challenge 2: Use DFS algorithm to do it.

Code - Java

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *   public int val;
 *   public TreeNode left, right;
 *   public TreeNode(int val) {
 *     this.val = val;
 *     this.left = this.right = null;
 *   }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Level order a list of lists of integer
     */
    public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<>();
        traverse(root, 0, result);
        Stack<ArrayList<Integer>> stack = new Stack<ArrayList<Integer>>();
        for (ArrayList<Integer> row : result) {
            stack.push(row);
        }
        result = new ArrayList<>();
        while (!stack.isEmpty()) {
            result.add(stack.pop());
        }
        return result;
    }

    private void traverse(TreeNode node, int level, ArrayList<ArrayList<Integer>> result) {
        if (node == null) return;

        while (level >= result.size()) {
            result.add(new ArrayList<Integer>());
        }
        result.get(level).add(node.val);
        traverse(node.left, level + 1, result);
        traverse(node.right, level + 1, result);
    }
}