### 题目

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);


### 直观解法

[3,1,3,2,3]

1 -> 1
2 -> 3
3 -> 0,2,4


### 蓄水池抽样（Reservoir Sampling）

当第一顶帽子来的时候：我只有1顶帽子，我以1/1的概率选择戴这顶帽子。

...



#### 代码

class Solution {

private int[] nums;
private Random r;

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

public int pick(int target) {
int res = -1;
int count = 0;
for (int i = 0; i < nums.length; i++) {
if (nums[i] == target) {
if (r.nextInt(++count) == count - 1) res = i;
}
}
return res;
}

}