Binary Tree Reverse Level Order Traversal- LeetCode: 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).

ReverseBinaryTreeTraversal.java

/**
 * 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>> levelOrderBottom(TreeNode root) {
          List<List<Integer>> result = new ArrayList<List<Integer>>();
    if(root==null) return result;
    Queue<TreeNode> q = new LinkedList<>();
    q.add(root);
    while(q.size()>0){
        List<Integer> list = new ArrayList<>();
        int size = q.size();
        for(int i=0; i<size; i++){
            TreeNode node = q.poll();
            list.add(node.val);
            if(node.left!=null) q.add(node.left);
            if(node.right!=null) q.add(node.right);
        }
        result.add(0,list);
    }
    return result;
}
}