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

### 题目描述

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

• 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

``````#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

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