Construct a Binary Tree from a String with Bracket Representation

class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left = None
        self.right = None

def str2tree(s: str):
    if not s:
        return None
    def helper(i):
        # Parse number (handle negative numbers)
        sign = 1
        if s[i] == '-':
            sign = -1
            i += 1
        num = 0
        while i < len(s) and s[i].isdigit():
            num = num * 10 + int(s[i])
            i += 1
        node = TreeNode(sign * num)
        # Parse left subtree if '(' found
        if i < len(s) and s[i] == '(':
            i += 1  # skip '('
            node.left, i = helper(i)
            i += 1  # skip ')'
        # Parse right subtree if '(' found
        if i < len(s) and s[i] == '(':
            i += 1  # skip '('
            node.right, i = helper(i)
            i += 1  # skip ')'
        return node, i
    root, _ = helper(0)
    return root

# Example usage:
if __name__ == '__main__':
    s = "4(2(3)(1))(6(5))"
    root = str2tree(s)
    # A function to print the tree can be added for verification
    print(root.val)