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[Leetcode]384. Shuffle an Array(C++)

题目描述

题目链接:384. Shuffle an Array

Given an integer array nums, design an algorithm to randomly shuffle the array.

Implement the Solution class:

  • Solution(int[] nums) Initializes the object with the integer array nums.
  • int[] reset() Resets the array to its original configuration and returns it.
  • int[] shuffle() Returns a random shuffling of the array.

例子

例子 1

Input:
["Solution", "shuffle", "reset", "shuffle"]
[[[1, 2, 3]], [], [], []]
Output:
[null, [3, 1, 2], [1, 2, 3], [1, 3, 2]]
Explanation:
Solution solution = new Solution([1, 2, 3]);
solution.shuffle(); // Shuffle the array [1,2,3] and return its result. Any permutation of [1,2,3] must be equally likely to be returned. Example: return [3, 1, 2]
solution.reset(); // Resets the array back to its original configuration [1,2,3]. Return [1, 2, 3]
solution.shuffle(); // Returns the random shuffling of array [1,2,3]. Example: return [1, 3, 2]

Constraints

  • 1 <= nums.length <= 200
  • -10^6 <= nums[i] <= 10^6
  • All the elements of nums are unique.
  • At most 5 * 10^4 calls will be made to reset and shuffle.

解题思路

这里用到了一个水塘采样的算法来实现随机排列 n 个元素,证明过程这里不再赘述,有兴趣可以查阅相关资料。具体做法是:

  • 从头开始遍历一遍数组
  • 在 i 个元素时,在其后 n - i 个元素中随机选取一个元素
  • 交换两元素

如此遍历一遍后则得到一个随机打乱的数组,代码如下:

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#include <vector>

class Solution {
public:
Solution(std::vector<int>& nums) : nums_(nums) {}

/** Resets the array to its original configuration and return it. */
std::vector<int> reset() { return nums_; }

/** Returns a random shuffling of the array. */
std::vector<int> shuffle() {
std::vector<int> ans = nums_;
for (int i = 0; i < ans.size(); ++i) {
int n = i + rand() % (ans.size() - i);
std::swap(ans[i], ans[n]);
}

return ans;
}

private:
std::vector<int> nums_;
};
  • 时间复杂度:
    • 构造函数: O(n)
    • reset(): O(1)
    • shuffule(): O(n)
  • 空间复杂度: O(n)

GitHub 代码同步地址: 384.ShuffleAnArray.cpp

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