LC.P470[用Rand7()实现Rand10()]

方法一：数学

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/**
 * 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 first, second;
        while ((first = rand7()) > 6); // 拒绝7，对[1,6]采样，把奇数和偶数作为采样结果，每种概率为0.5
        while ((second = rand7()) > 5); // 拒绝6,7，对[1,5]采样，有五种结果，每种概率为0.2
        return (first & 1) == 1 ? second : 5 + second;
    }
}