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LeetCode

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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]
]

Code

class 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();
            ArrayList<Integer> curr = new ArrayList<>();
            for(int i = 0; i < size; i++){
                TreeNode temp = queue.poll();
                curr.add(temp.val);
                if(temp.left != null){
                    queue.offer(temp.left);
                }

                if(temp.right != null){
                    queue.offer(temp.right);
                }
            }

            res.add(0, curr);
        }
        return res;
    }
}