leetcode 384- Shuffle an Array --STL shuffle算法的實現

384. Shuffle an Array

題目：
Shuffle a set of numbers without duplicates.

Example:

// Init an array with set 1, 2, and 3.
int[] nums = {1,2,3};
Solution solution = new Solution(nums);

// Shuffle the array [1,2,3] and return its result. Any permutation of [1,2,3] must equally likely to be returned.
solution.shuffle();

// Resets the array back to its original configuration [1,2,3].
solution.reset();

// Returns the random shuffling of array [1,2,3].
solution.shuffle();

思路：看到題目立刻就能想到STL中的random_shuffle算法，該算法原型爲：

voidrandom_shuffle(RandomAccessIteratorfirst,RandomAccessIteratorlast)

該算法將從N！種可能的數組排列序列中選擇一種，《The Art of Computer Programming 》3.4.2節給出了關於隨機排列的算法。假設需要隨機排列的序列爲X1，X2,……,Xt，算法描述如下：

1. 初始化 ，另j = t
2. 生成0~1之間均勻分佈的隨機數U
3. 另 k = j*U + 1（向下取整,k相當於1~j之間均勻分佈的隨機整數）,交換Xj和Xk

4. j = j - 1,如果j > 1,goto步驟2

   C++代碼描述如下：

random_shuffule(RandomAccessIterator first,RandomAccessIterator last){
    if(first == last)return;
    for(auto iter = first + 1;iter!=last;++iter)
        iter_swap(i,first + rand((i - first ) + 1));
}

解答時可直接使用STL算法：

class Solution {
public:
    Solution(vector<int> nums):data(nums) {
    }
    /** Resets the array to its original configuration and return it. */
    vector<int> reset() {
        return data;
    }
    /** Returns a random shuffling of the array. */
    vector<int> shuffle() {
        vector<int> res = data;
        random_shuffle(res.begin(),res.end());
        return res;
        }
vector<int> data;
};