[107. Binary Tree Level Order Traversal II] #leetcode

107_0828.java

class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> res = new LinkedList<List<Integer>>();
        if (root == null) return res;
        
        Deque<TreeNode> deque = new LinkedList<TreeNode>();
        deque.addLast(root);
        while ( !deque.isEmpty() ) {
            int n = deque.size();
            List<Integer> currentLevel = new ArrayList<Integer>();
            for ( int i = 0 ; i < n ; i++ ) {
                TreeNode node = deque.removeFirst();
                currentLevel.add(node.val);
                if ( node.left != null ) deque.addLast(node.left);
                if ( node.right != null ) deque.addLast(node.right);
            }
            res.add(0, currentLevel);
        }
        
        return res;
    }
}