384.Shuffle an Array

Tags: [random], [math]

Link: https://leetcode.com/problems/shuffle-an-array/#/description

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();

Solution: Random, Math

class Solution(object):

    def __init__(self, nums):
        """
        :type nums: List[int]
        """
        self.original_nums = nums

    def reset(self):
        """
        Resets the array to its original configuration and return it.
        :rtype: List[int]
        """
        return self.original_nums


    def shuffle(self):
        """
        Returns a random shuffling of the array.
        :rtype: List[int]
        """
        arr = list(self.original_nums)
        for i in xrange(len(arr) - 1):
            rand_index = random.randint(i, len(arr) - 1)
            arr[i], arr[rand_index] = arr[rand_index], arr[i]

        return arr

# Your Solution object will be instantiated and called as such:
# obj = Solution(nums)
# param_1 = obj.reset()
# param_2 = obj.shuffle()

Revelation:

• The proof and explanation of the shuffle algorithm is here: https://zhaonanli.gitbooks.io/algorithm/content/shuffle.html

Note:

• Time complexity of 'reset' = O(1).
• Time complexity of 'shuffle' = O(n), n is the numbers of the given elements.