384. Shuffle an 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();

解法一：

public class Solution {
    private int[] nums;
    private Random random;

    public Solution(int[] nums) {
        this.nums = nums;
        random = 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[] a = nums.clone();
        for(int j = 1; j < a.length; j++) {
            int i = random.nextInt(j + 1);
            swap(a, i, j);
        }
        return a;
    }
    
    private void swap(int[] a, int i, int j) {
        int t = a[i];
        a[i] = a[j];
        a[j] = t;
    }
}

解法2: 提前generate所有permutations，然后每次任意取一个。
如果用完一个，这次shuffle不能再用的话，就搞一下index array
但是超时！！！

 public class Solution {  
   int[] indices;  
   int end;  
   Random rand;  
   List<List<Integer>> candidates;  
   public Solution(int[] nums) {  
     int n = nums.length;  
     rand = new Random();  
     candidates = new ArrayList<>();  
     candidates.add(new ArrayList<>());  
     for(int i = 0; i < n; i++){  
       int size = candidates.size();  
       for(int j = 0; j < size;j++){  
         List<Integer> list = candidates.remove(0);  
         for(int k = 0; k < list.size();k++){  
           list.add(k,nums[i]);  
           candidates.add(new ArrayList<>(list));  
           list.remove(k);  
         }  
         list.add(nums[i]);  
         candidates.add(new ArrayList<>(list));  
       }  
     }  
     end = candidates.size()-1;  
     indices = new int[candidates.size()];  
     for(int i = 0; i < candidates.size(); i++)  
       indices[i] = i;  
   }  
   /** Resets the array to its original configuration and return it. */  
   public int[] reset() {  
     int n = indices.length;  
     for(int i = 0; i < n; i++)  
       indices[i] = i;  
     end = --n;  
     int[] ans = new int[candidates.get(end).size()];  
     int k = 0;      
     for(int num: candidates.get(end))  
       ans[k++] = num;  
     return ans;  
   }  
   /** Returns a random shuffling of the array. */  
   public int[] shuffle() {  
     if(end == -1) reset();  
     int i = rand.nextInt(end+1);  
     int idx = indices[i];  
     swap(indices, end, i);  
     end--;  
     int[] ans = new int[candidates.get(idx).size()];  
     int k = 0;  
     for(int num: candidates.get(idx))  
       ans[k++] = num;  
     return ans;  
   }  
   public void swap(int[] A, int i, int j){  
     int tmp = A[i];  
     A[i] = A[j];  
     A[j] = tmp;  
   }  
 }  
 /**  
  * Your Solution object will be instantiated and called as such:  
  * Solution obj = new Solution(nums);  
  * int[] param_1 = obj.reset();  
  * int[] param_2 = obj.shuffle();  
  */