0001 uint64_t power(uint64_t a, int n) { //a^n
0002    uint64_t pow = 1, p = a;
0003    while (0 < n) { //O(logn)
0004       if (n & 1) pow *= p; //O(1)？
0005       n >>= 1; p *= p; //O(1)？
0006    } //while
0007    return pow;
0008 } //高效版，O(logn)