Guess Number Higher or Lower Problem & Solution

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.

See the guess number higher or lower problem on LeetCode.

C++ Solution

The runtime complexity of the algorithm is O ( log n ) O(\log{}n) .

#pragma GCC optimize("Ofast")
#pragma GCC optimization("unroll-loops")

static const int _=[](){std::ios::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);return 0;}();

/**
 * Forward declaration of guess API.
 * @param  num   your guess
 * @return 	     -1 if num is lower than the guess number
 *			         1 if num is higher than the guess number
 *               otherwise return 0
 * int guess(int num);
 */
class Solution {
public:
  int guessNumber(int n) {
    int low = 1;
    int high = n;
    while (low < high) {
      int mid = low + (high - low) / 2;
      int result = guess(mid);

      if (result == 0) {
        return mid;
      } else if (result > 0) {
        low = mid + 1;
      } else {
        high = mid;
      }
    }
    
    return low;
  }
};

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