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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:

/**
 * 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

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