# Leetcode Algorithms

398. Random Pick Index

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);

Logic:

Create a random set, and then iterate through the set. If the value of the set is equal to target, increment count.

Solution:

class Solution {

private Random rand;

private int[] nums;

private int k = 1;public Solution(int[] nums) {

rand = new Random();

this.nums = nums;

}

public int pick(int target) {

int result = -1;

int count = 0;

for (int i = 0; i < nums.length; i++){

if (nums[i] == target){

count ++;

if (rand.nextInt(count) < k){

result = i;

}

}

}

return result;

}

};