374. Guess Number Higher or Lower

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problem

solution

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

class Solution {
public:
    int guessNumber(int n) {
        int l = 1, r = n;
        while(l<=r){
            int mid = l + (r-l)/2;
            int ans = guess(mid);
            if( ans == 0) return mid;
            else if(ans == -1) r = mid-1;
            else l = mid+1;
        }
        return l;
        
    }
};

analysis

  • time complexity O(logn)
  • space complexity O(1)