107. Binary Tree Level Order Traversal II

Posted 2020-07-07 我的名字叫周周

    /*
     * 107. Binary Tree Level Order Traversal II 
     * 11.21 By Mingyang I的解答加一行：
     * Collections.reverse(results);就好了
     */
        public List<List<Integer>> levelOrderBottom(TreeNode root) {
            List<List<Integer>> results = new ArrayList<List<Integer>>();
            ArrayList<Integer> result = new ArrayList<Integer>();
            if (root == null)
                return results;
            LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
            queue.add(root);
            int currentNumber = 1;
            int nextNumber = 0;
            while (queue.size() != 0) {
                TreeNode temp = queue.poll();
                result.add(temp.val);
                currentNumber--;
                if (temp.left != null) {
                    queue.add(temp.left);
                    nextNumber++;
                }
                if (temp.right != null) {
                    queue.add(temp.right);
                    nextNumber++;
                }
                if (currentNumber == 0) {
                    results.add(result);
                    currentNumber = nextNumber;
                    nextNumber = 0;
                    result = new ArrayList<Integer>();
                }
            }
         Collections.reverse(results);
         return results;
    }