374.Guess Number Higher or Lower

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Binary Search, Easy

Question

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):

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

Example:

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n = 10, I pick 6.
Return 6.

Answer

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/* The guess API is defined in the parent class GuessGame.
   @param num, your guess
   @return -1 if my number is lower, 1 if my number is higher, otherwise return 0
      int guess(int num); */
public class Solution extends GuessGame {
    public int guessNumber(int n) {
        int start = 1, end = n;
        while(start < end){
            //int mid = (start + end) / 2;
            int mid = i + (j - i) / 2;
            if(guess(mid)==0)
                return mid;
            else if(guess(mid)==1)
                start = mid + 1;
            else
                end = mid;       
        }
        return start;
    }
}

Time complexity: O(logn)

Space complexity: O(1)

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int mid = (start + end) / 2;

it will become infinite loop

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1063376695
-552418605
787167393
-690523256
718115067
-725049419
700851986
-733680959
696536216
-735838844
695457273
-736378316
695187537
-736513184
695120103
-736546901
695103245
-736555330
695099030
...

due to integer overflow (when j+i becomes a negative number), you get into an infinite loop.