374. Guess Number Higher or Lower-猜数字大小

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 will tell you whether the number I picked is higher or lower than your guess.

You call a pre-defined API int guess(int num), which returns 3 possible results:

• -1: The number I picked is lower than your guess (i.e. pick < num).
• 1: The number I picked is higher than your guess (i.e. pick > num).
• 0: The number I picked is equal to your guess (i.e. pick == num).
  Return the number that I picked.

Example 1:
Input: n = 10, pick = 6
Output: 6

Example 2:
Input: n = 1, pick = 1
Output: 1

Example 3:
Input: n = 2, pick = 1
Output: 1

Example 4:
Input: n = 2, pick = 2
Output: 2

Constraints:
1 <= n <= 2^31 - 1
1 <= pick <= n

Solution in python:

# 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: int) -> int:

class Solution:
    def guessNumber(self, n: int) -> int:

        def helper(left, right):
            mid = (left+right) // 2
            value = guess(mid)
            if value == -1:
                return helper(left, mid-1)
            elif value == 1:
                return helper(mid+1, right)
            else:
                return mid
        return helper(1, n)