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[Leetcode]374. Guess Number Higher or Lower(C++)

题目描述

题目链接: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.

例子

例子 1

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

例子 2

Input: n = 1, pick = 1
Output: 1

例子 3

Input: n = 2, pick = 1
Output: 1

Constraints

  • 1 <= n <= 2^31 - 1
  • 1 <= pick <= n

解题思路

在给定范围内猜一个数,用简单的二分查找即可。每次猜错时表明中间数不在搜索范围,因此收缩左/右边界时应该收缩到中间值右/左移一位。代码如下:

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

return left;
}
};
  • 时间复杂度: O(log n)
  • 空间复杂度: O(1)

GitHub 代码同步地址: 374.GuessNumberHigherOrLower.cpp

其他题目:
GitHub: Leetcode-C++-Solution
博客: Leetcode-Solutions