Given an array of integers with possible duplicates, randomly output the index of a given target number. You can assume that the given target number must exist in the array.
Note:
The array size can be very large. Solution that uses too much extra space will not pass the judge.
Example:
int[] nums = new int[] {1,2,3,3,3};
Solution solution = new Solution(nums);
// pick(3) should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.
solution.pick(3);
// pick(1) should return 0. Since in the array only nums[0] is equal to 1.
solution.pick(1);
解法：Reservoir Sampling
public class ReservoirSamplingTest {
private int[] pool; // 所有数据
private final int N = 100000; // 数据规模
private Random random = new Random();
@Before
public void setUp() throws Exception {
// 初始化
pool = new int[N];
for (int i = 0; i < N; i++) {
pool[i] = i;
}
}
private int[] sampling(int K) {
int[] result = new int[K];
for (int i = 0; i < K; i++) { // 前 K 个元素直接放入数组中
result[i] = pool[i];
}
for (int i = K; i < N; i++) { // K + 1 个元素开始进行概率采样
int r = random.nextInt(i + 1);
if (r < K) {
result[r] = pool[i];
}
}
return result;
}
@Test
public void test() throws Exception {
for (int i : sampling(100)) {
System.out.println(i);
}
}
}
