function countSolutions(s) {
    // Helper function to resolve a "?" to either "P" or "_"
    function set(list, i, ch) {
        if (list[i] == "?") list[i] = ch; // Resolve "?"
        return (list[i] == "P") == (ch == "P"); // Is false when there is a contradiction
    }
    
    // Main algorithm for a single sweep from left to right
    function sweep(list) {
        let count = 0;
        for (let i = 2; i < list.length; i++) {
            switch (list[i-1]) {
            case "0":
                if (!set(list, i, "_")) return 0; // Contradiction
                break;
            case "2":
                if (!set(list, i, "P")) return 0; // Contradiction
                break;
            case "1":
                switch (list[i-2]) {
                case "?": 
                    count--; // Undo. Take into account the chain-effect
                    break;
                case "P":
                    if (!set(list, i, "_")) return 0; // Contradiction
                    break;
                default: // "0", "1", or "_"
                    if (!set(list, i, "P")) return 0; // Contradiction
                    break;
                }
                break;
            case "?":
                count++;
            }                
        }
        return 1 << count;
    }

    // Convert character string to array of characters (so it is mutable)
    //    To ease the process, add two extra characters (denoting no pitholes)
    let list = Array.from("_" + s + "_");
    // Perform two sweeps in opposite directions. If the first returns 0
    //    then don't bother with the second sweep as there is no solution.
    return sweep(list) && sweep(list.reverse());
}

// Execute the algorithm for the two test cases given in the question
const tests = [
    "?01???",
    "PP12"
]

for (const test of tests) {
    // Output both the test and the result for it
    console.log(test, countSolutions(test));
}