LeetCode 374. Guess Number Higher or Lower

Description

https://leetcode.com/problems/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 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 <= 231 - 1
  • 1 <= pick <= n

Explanation

Do a binary search to find the pick.

Python Solution

# 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:
        
        start = 1
        end = n
        
        while start + 1 < end:
            mid = start + (end - start) // 2
            if guess(mid) == 0:
                return mid
            elif guess(mid) == -1:
                end = mid
            else:
                start = mid
                
        if guess(end) == 0:
            return end
        
        if guess(start) == 0:
            return start
        
  • Time Complexity: O(log(N)).
  • Space Complexity: O(1).

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