LEVEL: 189 sol = solution(45, 516, 4, '+10', 'mirror', 'reverse', [4, 2])
operator list:
'-3': minues 3
'+5': plus 5
'x2': times 2
'/3': divides 3
10: add '10' at the end of current number
'+/-': change sign
'<<': backspace
'reverse': reverse number, keep negetive sign ('-') if it exists
'mirror': 32 -> 2332
'5=>13': replace
'shift>': 231 -> 123
'<shift': -150 -> -501
'[+]2': increase each number of operators 2 (e.g. +3 -> +5)
'sum': 1023 -> (1+0+2+3) -> 6
'x^3': power
'inv10': 1250 -> (10-1, 10-8, 10-5, 0) -> 9850
'store': store current result as a number operator (ignore negetive)
[4, 2]: portal indicator, 4th digital will added to the 2nd digital
1123 -> (123 + 10) -> 133