374. Guess Number Higher or Lower

• Difficulty: Easy

• Topics: Binary Search

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Problem:

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 :

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

Solutions:

// Forward declaration of guess API.
// @param num, your guess
// @return -1 if my number is lower, 1 if my number is higher, otherwise return 0
int guess(int num);

class Solution {
public:
    int guessNumber(int n) {
        int left = 1;
        int right = n;
        while (left < right) {
            int mid = left + (right - left) / 2;
            int ret = guess(mid);
            if (ret == 0)   return mid;
            else if (ret == 1) {
                left = mid + 1;
            } else {
                right = mid - 1;
            }
        }

        return left;
    }
};