题目：

题解：

// The rand7() API is already defined for you.
// int rand7();
// @return a random integer in the range 1 to 7

int rand10() {
    while(true) {
        int index = (rand7() - 1) * 7 + rand7(); 
        if(index <= 40) return index % 10 + 1; 
    }
}