[606. Construct String from Binary Tree] #leetcode

606_0902.java

class Solution {
    public String tree2str(TreeNode t) {
        if (t == null) return "";
        String left = tree2str(t.left);
        String right = tree2str(t.right);
        
        StringBuilder sb = new StringBuilder();
        sb.append(t.val);
        if (right.equals("")) {
            if (!left.equals("")) sb.append("(" + left + ")");
        } else {
            sb.append("(" + left + ")");
            sb.append("(" + right + ")"); 
        }
        return sb.toString();
    }
}