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public class Main {
    public static void main(String[] args) {
        String str = "100?10?1010?000";
        System.out.println("All possible combinations:");
        generateCombinations(str.toCharArray(), 0);  // Start at the first index
    }

    // Recursive function to generate combinations
    public static void generateCombinations(char[] str, int index) {
        // Base case: If we reach the end of the string, print the result
        if (index == str.length) {
            System.out.println(new String(str));  // Print the current combination
            return;
        }

        // If the current character is '?', replace it with '0' and '1'
        if (str[index] == '?') {
            // Replace with '0' and recurse
            str[index] = '0';
            generateCombinations(str, index + 1);

            // Replace with '1' and recurse
            str[index] = '1';
            generateCombinations(str, index + 1);

            // Backtrack: Set it back to '?' to explore other branches
            str[index] = '?';
        } else {
            // If the current character is not '?', just move to the next character
            generateCombinations(str, index + 1);
        }
    }
}