You are given an integer array cards of length 4. You have four cards, each containing a number in the range [1, 9]. You should arrange the numbers on these cards in a mathematical expression using the operators ['+', '-', '', '/'] and the parentheses '(' and ')' to get the value 24. You are restricted with the following rules: The division operator '/' represents real division, not integer division. For example, 4 / (1 - 2 / 3) = 4 / (1 / 3) = 12. Every operation done is between two numbers. In particular, we cannot use '-' as a unary operator. For example, if cards = [1, 1, 1, 1], the expression "-1 - 1 - 1 - 1 " is not allowed. You cannot concatenate numbers together For example, if cards = [1, 2, 1, 2], the expression "12 + 12 " is not valid.