LintCode - 두 갈래 트리 레이아웃 II (중간)

  {3,9,20,#,#,15,7},
    3
   / \
  9  20
    /  \
   15   7

 ：
[
  [15,7],
  [9,20],
  [3]
]

/**
 * 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: A tree
     * @return: buttom-up level order a list of lists of integer
     */
    public List> levelOrderBottom(TreeNode root) {
        List> ret = new ArrayList>();
        if (root == null) {
            return ret;
        }

        List cell = new ArrayList();
        TreeNode last = root;
        TreeNode nLast = root;

        LinkedList queue = new LinkedList();
        queue.add(root);
        while (!queue.isEmpty()) {
            TreeNode node = queue.pop();
            cell.add(node.val);

            if (node.left != null) {
                queue.add(node.left);
                nLast = node.left;
            }

            if (node.right != null) {
                queue.add(node.right);
                nLast = node.right;
            }

            // 
            if (node == last) {
                last = nLast;
                ret.add(0, cell);
                cell = new ArrayList();
            }
        }
        return ret;
    }
}