374. Guess Number Higher or Lower *

We are playing the Guess Game. The game is as follows:

I pick a number from 1 to n. You have to guess which number I picked.

Every time you guess wrong, I'll tell you whether the number is higher or lower.

You call a pre-defined API 

guess(int num)

 which returns 3 possible results (

-1

, 

1

,
or 

0

):

-1 : My number is lower
1 : My number is higher
0 : Congrats! You got it!

Example:

n = 10, I pick 6.

Return 6.

My code: 40ms 0.86

# The guess API is already defined for you.
# @param num, your guess
# @return -1 if my number is lower, 1 if my number is higher, otherwise return 0
# def guess(num):

class Solution(object):
def guessNumber(self, n):
"""
:type n: int
:rtype: int
"""
guess_num = n/2
high_index = n
low_index = 0
flag = guess(guess_num)
while high_index-low_index !=1:
if flag ==-1:
high_index = guess_num
else:
low_index = guess_num
guess_num = low_index +(high_index-low_index)/2
flag = guess(guess_num)
if flag == 0 :
return guess_num
if flag ==0:
return guess_num
else:
return high_index

Traps:

   It's efficient to use binary search method in this problem.However, when the difference between high_index and low_index is 1, the code would loop indefinitely.

Questions:

 How to understand the overflow problem ?

 Is it essential to judge low_index<high_index every time?