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## Problem

Given a singly linked list, return a random node’s value from the linked list. Each node must have the same probability of being chosen.

Example 1:

Input
["Solution", "getRandom", "getRandom", "getRandom", "getRandom", "getRandom"]
[[[1, 2, 3]], [], [], [], [], []]
Output
[null, 1, 3, 2, 2, 3]

Explanation
Solution solution = new Solution([1, 2, 3]);
solution.getRandom(); // return 1
solution.getRandom(); // return 3
solution.getRandom(); // return 2
solution.getRandom(); // return 2
solution.getRandom(); // return 3
// getRandom() should return either 1, 2, or 3 randomly. Each element should have equal probability of returning.


• What if the linked list is extremely large and its length is unknown to you?
• Could you solve this efficiently without using extra space?

## Code

Reservoir sampling (水塘抽样)

class Solution {
Random rand;
rand = new Random();
}

public int getRandom() {
int selected = -1;
int i = 1;

while(temp != null){
if(rand.nextFloat() < 1.0 / i){
selected = temp.val;
}
temp = temp.next;
i++;
}

return selected;
}
}