public static void print(int n) {
    if ( n <= 0) return;

    char[] number = new char[n];
    print1ToMax(number, 0);
}
//digit代表位数
public static void print1ToMax(char[] number, int digit) {
    if (digit == number.length) {   //地柜结束条件，到叶子节点
        printNumber(number);
        return;
    }

    for (int i = 0; i < 10; i++) {
        number[digit] = (char) (i + '0');
        print1ToMax(number, digit + 1);
    }
}

public static void printNumber(char[] number) {
    int index = 0;
    while (index < number.length && number[index] == '0') {
        index++;
    }
    while (index < number.length) {
        System.out.print(number[index++]);
    }

    System.out.println();
}