Binary search

二分法。

一般 pattern：

low = 0
high = n
while low <= high:
    mid = (low + high) // 2
    mid_calc = f(mid)
    if mid_calc == target:
        return g(mid)
    elif mid_calc < target:
        low = mid + 1
    else:
        high = mid - 1
return default_retval

注意 low <= high, low = mid + 1 和 high = mid - 1。