/*
1. 扩展rand 范围: 
rand7() :           1,2, 3, 4, 5, 6, 7
(rand7()-1) * 7 :   0,7,14,21,28,35,42
(rand7()-1) * 7 + rand7() : 相加元素无重复, 即等概率构成了 1~49, 即rand49()


2. 缩小rand 范围:
2.1 拒绝采样缩小为10的倍数:  r40 = r49 <= 40 ? r49 : rand10()
2.2 10的倍数缩小为10:  r10 = r40 % 10 + 1

*/

/**
 * The rand7() API is already defined in the parent class SolBase.
 * public int rand7();
 * @return a random integer in the range 1 to 7
 */
class Solution extends SolBase {
    public int rand10() {
        int r49 = (rand7() - 1 ) * 7 + rand7();
        return r49 <= 40 ? r49 % 10 + 1 : rand10();
    }
}