L107. Binary Tree Level Order Traversal II

Problem:

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,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]

Solution:

public List<List<Integer> levelOrderBottom(TreeNode root){
    List<List<Integer>> res = new ArrayList<>();
    if(root == null) return res;
    
    Queue<TreeNode> queue = new LinkedList<>();
    queue.offer(root);
    
    while(!queue.isEmpty()){
        int size = queue.size();
        List<Integer> list = new ArrayList<>();
        for(int i = 0; i < queue.size(); i++){
            TreeNode node = queue.poll();
            list.add(node.val);
            if(node.left != null){
                queue.offer(node.left);
            }
            if(node.right != null){
                queue.offer(node.right);
            }
        }
        res.add(list);
    }
    Collections.reverse(res);
    return res;
}