​LeetCode刷题实战606：根据二叉树创建字符串

class Solution {
    StringBuilder sb = new StringBuilder();
    public String tree2str(TreeNode root) {
        dfs(root);
        return sb.substring(1, sb.length() - 1);
    }
    void dfs(TreeNode root) {
        sb.append("(");
        sb.append(root.val);
        if (root.left != null) dfs(root.left);
        else if (root.right != null) sb.append("()");
        if (root.right != null) dfs(root.right);
        sb.append(")"); 
    }
}