398. Random Pick Index (Medium)

https://leetcode.com/problems/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);

Hints

Solutions

  • Java

https://www.cnblogs.com/grandyang/p/5875509.html

class Solution {

    private int[] nums;

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

    public int pick(int target) {
        int count = 0;
        for (int num : nums) {
            if (num == target) {
                count++;
            }
        }

        int pick = (int)(Math.random()*count);

        int idx = 0;
        count = 0;
        for(int i = 0; i < nums.length; i ++){
            if(nums[i] != target) continue;

            if (count++ == pick) {
                idx = i;    
                break;
            }
        }

        return idx;
    }
}
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