Binary Tree Reverse Level Order Traversal

ReverseLevelOrderTraversal.java

/*
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]
]
*/

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        
        if(root == null) {
            return new LinkedList<List<Integer>>();
        }
        
        List<List<Integer>> l = new LinkedList<List<Integer>>();
        List<Integer> currentLevel = new LinkedList<Integer>();

        Queue<TreeNode> q = new LinkedList<TreeNode>();
        int prevN = 1;
        int currentN = 0;
        q.add(root);
        while(!q.isEmpty()){
            
            TreeNode node = q.poll();
            currentLevel.add(node.val);
            prevN--;
            
            if(node.left != null) {
                currentN++;
                q.add(node.left);
            }     
            if(node.right != null) {
                currentN++;
                q.add(node.right);
            }
            
            if(prevN == 0) {
                l.add(0,currentLevel);
                currentLevel = new LinkedList<Integer>();
                prevN = currentN;
                currentN = 0;
            }
        }
        if(currentLevel.size() != 0 ){
            l.add(0,currentLevel);
        }
        
        return l;
    }
}