[LeetCode] 374. 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

猜数字大小。

猜数字游戏的规则如下：

每轮游戏，我都会从 1 到 n 随机选择一个数字。 请你猜选出的是哪个数字。
如果你猜错了，我会告诉你，你猜测的数字比我选出的数字是大了还是小了。
你可以通过调用一个预先定义好的接口 int guess(int num) 来获取猜测结果，返回值一共有 3 种可能的情况（-1，1 或 0）：

-1：我选出的数字比你猜的数字小 pick < num
1：我选出的数字比你猜的数字大 pick > num
0：我选出的数字和你猜的数字一样。恭喜！你猜对了！pick == num
返回我选出的数字。

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/guess-number-higher-or-lower
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

这也是一个通过二分法调用API猜数字大小的题目。注意判断的边界是从1到N。

时间O(logn)

空间O(1)

/* 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;
        int end = n;
        // start + 1 < end
        // 举例，start - 0, end = 3
        // 中间隔了起码有start + 1和start + 2两个下标
        // 这样跳出while循环的时候，end < start
        // 才有了最后的两个判断
        while (start + 1 < end) {
            int mid = start + (end - start) / 2;
            if (guess(mid) == 0) {
                return mid;
            } else if (guess(mid) == 1) {
                start = mid;
            } else {
                end = mid;
            }
        }

        if (guess(start) == 0) {
            return start;
        } else {
            return end;
        }
    }
}

另一种二分模板的写法，注意while的条件和start, end指针的变化。

/* 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;
        int end = n;
        while (start <= end) {
            int mid = start + (end - start) / 2;
            if (guess(mid) == 0) {
                return mid;
            } else if (guess(mid) == 1) {
                start = mid + 1;
            } else {
                end = mid - 1;
            }
        }
        return start;
    }
}

LeetCode 题目总结