103. Binary Tree Zigzag Level Order Traversal

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
  public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
    if (root == null) {
      return new ArrayList<List<Integer>>();
    }

    List<List<Integer>> results = new ArrayList<List<Integer>>();

    // add the root element with a delimiter to kick off the BFS loop
    LinkedList<TreeNode> node_queue = new LinkedList<TreeNode>();
    node_queue.addLast(root);
    node_queue.addLast(null);

    LinkedList<Integer> level_list = new LinkedList<Integer>();
    boolean is_order_left = true;

    while (node_queue.size() > 0) {
      TreeNode curr_node = node_queue.pollFirst();
      if (curr_node != null) {
        if (is_order_left)
          level_list.addLast(curr_node.val);
        else
          level_list.addFirst(curr_node.val);

        if (curr_node.left != null)
          node_queue.addLast(curr_node.left);
        if (curr_node.right != null)
          node_queue.addLast(curr_node.right);

      } else {
        // we finish the scan of one level
        results.add(level_list);
        level_list = new LinkedList<Integer>();
        // prepare for the next level
        if (node_queue.size() > 0)
          node_queue.addLast(null);
        is_order_left = !is_order_left;
      }
    }
    return results;
  }
}