题目：

题解：

class Solution {
public:
    int rand10() {
        int a, b, idx;
        while (true) {
            a = rand7();
            b = rand7();
            idx = b + (a - 1) * 7;
            if (idx <= 40) {
                return 1 + (idx - 1) % 10;
            }
            a = idx - 40;
            b = rand7();
            // get uniform dist from 1 - 63
            idx = b + (a - 1) * 7;
            if (idx <= 60) {
                return 1 + (idx - 1) % 10;
            }
            a = idx - 60;
            b = rand7();
            // get uniform dist from 1 - 21
            idx = b + (a - 1) * 7;
            if (idx <= 20) {
                return 1 + (idx - 1) % 10;
            }
        }
    }
};