random-pick-index.js

/**
 * 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);
 *
 * Your Solution object will be instantiated and called as such:
 * var obj = Object.create(Solution).createNew(nums)
 * var param_1 = obj.pick(target)
 */

class Solution {
  /**
   * @param {number[]} nums
   */
  constructor(nums) {
    this.nums = nums;
  }

  /**
   * @param {number} target
   * @return {number}
   */
  pick(target) {
    let count = 0;
    let res = null;

    for (let i = 0; i < this.nums.length; i++) {
      if (this.nums[i] === target) {
        count++;
        if (randomInt(0, count) === count - 1) {
          res = i;
        }
      }
    }

    return res;
  }
}

export { Solution };