1. class Solution {
    2. public:
    3.     int rand10() {
    4.         int a, b, idx;
    5.         while (true) {
    6.             a = rand7();
    7.             b = rand7();
    8.             idx = b + (a - 1) * 7;
    9.             if (idx <= 40)
    10.                 return 1 + (idx - 1) % 10;
    11.             a = idx - 40;
    12.             b = rand7();
    13.             // get uniform dist from 1 - 63
    14.             idx = b + (a - 1) * 7;
    15.             if (idx <= 60)
    16.                 return 1 + (idx - 1) % 10;
    17.             a = idx - 60;
    18.             b = rand7();
    19.             // get uniform dist from 1 - 21
    20.             idx = b + (a - 1) * 7;
    21.             if (idx <= 20)
    22.                 return 1 + (idx - 1) % 10;
    23.         }
    24.     }
    25. };