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/**
 * Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).
 * For example:
 * Given binary tree {3,9,20,#,#,15,7},
 *     3
       / \
      9  20
        /  \
       15   7
 * return its bottom-up level order traversal as:
 * [
      [15,7]
      [9,20],
      [3],
    ]
 */
 public class Solution {
    public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
        if (root == null)
			return res;
		// Initialization
		Queue<TreeNode> q = new LinkedList<TreeNode>();
		q.offer(root);
		q.offer(null);
        Stack<ArrayList<Integer>> auxilary_s = new Stack<ArrayList<Integer>>();
		ArrayList<Integer> curr = new ArrayList<Integer>();
		while (!q.isEmpty()) {
			TreeNode tmp = q.poll();
			if (tmp != null) {
				curr.add(tmp.val);
				if (tmp.left != null)
					q.offer(tmp.left);
				if (tmp.right != null)
					q.offer(tmp.right);
			} else {
				ArrayList<Integer> c_curr = new ArrayList<Integer>(curr);
				auxilary_s.add(c_curr);
				curr.clear(); // Java will clear the reference, so have to new an new ArrayList. 
				// completion of a level;
				if (!q.isEmpty())
					q.offer(null);

			}
		}
        while(!auxilary_s.isEmpty()){
            res.add(auxilary_s.pop());
        }
		return res;
    }
}

/**
 * Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).
 * For 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]
    ]
 */
 /**
 * 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) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
    	if (root == null)
			return res;
		// Initialization
		Queue<TreeNode> q = new LinkedList<TreeNode>();
		q.offer(root);
		q.offer(null);
		ArrayList<Integer> curr = new ArrayList<Integer>();
		while (!q.isEmpty()) {
			TreeNode tmp = q.poll();
			if (tmp != null) {
				curr.add(tmp.val);
				if (tmp.left != null)
					q.offer(tmp.left);
				if (tmp.right != null)
					q.offer(tmp.right);
			} else {
				ArrayList<Integer> c_curr = new ArrayList<Integer>(curr);
				res.add(c_curr);
				curr.clear(); // Java will clear the reference, so have to new an new ArrayList. 
				// completion of a level;
				if (!q.isEmpty())
					q.offer(null);
			}
		}
		return res;
    }
 }