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public List<String> binaryTreePaths(TreeNode root) {
            List<String> ans = new ArrayList<>();
            if (root == null) {
                return ans;
            }
            StringBuilder sb = new StringBuilder();
            backtrack(root, sb, ans);
            return ans;
        }

        private void backtrack(TreeNode root, StringBuilder sb, List<String> ans) {
            if (root == null) {
                return;
            }
            // 记录当前拼接前的长度，用于回溯撤销
            int len = sb.length();

            sb.append(root.val);
            // 到达叶子节点，添加完整路径
            if (root.left == null && root.right == null) {
                ans.add(sb.toString());
            } else {
                sb.append("->");
                backtrack(root.left, sb, ans);
                backtrack(root.right, sb, ans);
            }
            // 回溯撤销，恢复StringBuilder到拼接前的状态
            sb.setLength(len);
        }