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;
    }
};

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