implement reservoir sampling algorithm

ReservoirSampling/ReservoirSampling.java

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+/* Problem:
+ *      From an array or stream with N size (the N can be very large or stream can be endless), 
+ *      choose k (k < N) elements randomly from array or stream.
+ *      Make sure each element is selected with same probability k/N
+ * */
+
+import java.util.*;
+public class ReservoirSampling{
+    public int[] sample(int[] nums, int k){
+        int[] res = new int[k];
+        for(int i = 0; i < k; ++i){
+            res[i] = nums[i];
+        }
+        Random rand = new Random();
+        int index = 0;
+        for(int i = k; i < nums.length; ++i){
+            index = rand.nextInt(i + 1); // [0, i+1) prob = k/(k + 1) * (k + 1)/(k + 2) * ... (N - 1)/N = k/N
+            if(index < k){
+                res[index] = nums[i];    // swap the element
+            }
+        }
+        return res;
+    }
+}