470. 用 Rand7() 实现 Rand10()

var rand10 = function() {
    var row, col, idx;
    do {
        row = rand7();
        col = rand7();
        idx = col + (row - 1) * 7;
    } while (idx > 40);
    return 1 + (idx - 1) % 10;
};

https://leetcode-cn.com/problems/implement-rand10-using-rand7/solution/yong-rand7-shi-xian-rand10-by-leetcode-s-qbmd/