Friday, August 12, 2016

LeetCode 384 - Shuffle an Array


https://www.hrwhisper.me/leetcode-shuffle-array/
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:用swap,每次从[i,n-1]中随机一个数,和第i个数交换即可。
当然,上面的代码是每次洗牌从上一次的结果继续进行的,这也符合我们对于洗牌的感觉。
如果每次从初始数组进行洗牌,那么reset数组只需要返回初始数组。。
    private int[] nums;
    private int[] output;
    private Random random;
    
    public Solution(int[] nums) {
        this.nums = nums;
        this.output = Arrays.copyOf(nums,nums.length);
        this.random = new Random();
    }
    
    /** Resets the array to its original configuration and return it. */
    public int[] reset() {
        return this.output = Arrays.copyOf(nums,nums.length);
    }
    
    /** Returns a random shuffling of the array. */
    public int[] shuffle() {
        int n = output.length;
for (int i = 0; i < n; i++) {
int _id = random.nextInt(n-i);
int temp = output[i];
output[i] = output[i+_id];
output[i+_id] = temp;
}
return output;
    }

http://www.learn4master.com/interview-questions/leetcode/leetcode-shuffle-an-array-java
    int[] origin;
    int[] res;
    Random r;
    public Solution(int[] nums) {
        origin = nums;
        res = Arrays.copyOf(nums, nums.length);
        r = new Random();
    }
    
    /** Resets the array to its original configuration and return it. */
    public int[] reset() {
        res = Arrays.copyOf(origin, origin.length);
        return res;
    }
    
    /** Returns a random shuffling of the array. */
    public int[] shuffle() {
        int right = res.length - 1;
        int j;
        
        while(right > 0) {
            j = r.nextInt(right + 1);
            swap(res, j, right);
            right--;
        }
        return res;
    }
    
    void swap(int[] A, int i, int j) {
        int tmp = A[i];
        A[i] = A[j];
        A[j] = tmp;
    }

http://www.guoting.org/leetcode/leetcode-384-shuffle-an-array/
    private int[] nums;
    private Random rand;

    public Solution(int[] nums){
        this.nums=nums;
        this.rand=new Random();
    }

    /** Resets the array to its original configuration and return it. */
    public int[] reset(){
        return nums;
    }

    /** Returns a random shuffling of the array. */
    public int[] shuffle(){
        if(nums==null) return null;
        int[] arr=nums.clone();
        for(int i=1;i<arr.length;i++) {
            int j=rand.nextInt(i+1);
            swap(arr,i,j);
        }
        return arr;
    }

    private void swap(int[] arr,int i,int j){
        int tmp=arr[i];
        arr[i]=arr[j];
        arr[j]=tmp;
    }
http://www.cnblogs.com/grandyang/p/5783392.html
这道题让我们给数组洗牌,也就是随机打乱顺序,那么由于之前那道题Linked List Random Node我们接触到了水塘抽样的思想,这道题实际上这道题也是用类似的思路,我们遍历数组每个位置,每次都随机生成一个坐标位置,然后交换当前遍历位置和随机生成的坐标位置的数字,这样如果数组有n个数字,那么我们也随机交换了n组位置,从而达到了洗牌的目的
    Solution(vector<int> nums): v(nums) {}
    
    /** Resets the array to its original configuration and return it. */
    vector<int> reset() {
        return v;
    }
    
    /** Returns a random shuffling of the array. */
    vector<int> shuffle() {
        vector<int> res = v;
        for (int i = 0; i < res.size(); ++i) {
            int t = rand() % res.size();
            swap(res[i], res[t]);
        }
        return res;
    }



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