Leetcode•Aug 05, 2025

Random Pick with Weight

Hazrat Ali

Leetcode

You need to implement the function pickIndex(), which randomly picks an index in the range [0, w.length - 1] (inclusive) and returns it. The probability of picking an index i is w[i] / sum(w).

  • For example, if w = [1, 3], the probability of picking index 0 is 1 / (1 + 3) = 0.25 (i.e., 25%), and the probability of picking index 1 is 3 / (1 + 3) = 0.75 (i.e., 75%).

Example 1:

Input
["Solution","pickIndex"]
[[[1]],[]]
Output
[null,0]

Explanation
Solution solution = new Solution([1]);
solution.pickIndex(); // return 0. The only option is to return 0 since there is only one element in w.

Example 2:

Input
["Solution","pickIndex","pickIndex","pickIndex","pickIndex","pickIndex"]
[[[1,3]],[],[],[],[],[]]
Output
[null,1,1,1,1,0]

Explanation
Solution solution = new Solution([1, 3]);
solution.pickIndex(); // return 1. It is returning the second element (index = 1) that has a probability of 3/4.
solution.pickIndex(); // return 1
solution.pickIndex(); // return 1
solution.pickIndex(); // return 1
solution.pickIndex(); // return 0. It is returning the first element (index = 0) that has a probability of 1/4.

Since this is a randomization problem, multiple answers are allowed.
All of the following outputs can be considered correct:
[null,1,1,1,1,0]
[null,1,1,1,1,1]
[null,1,1,1,0,0]
[null,1,1,1,0,1]
[null,1,0,1,0,0]
......
and so on.


Solution

const getRandomArbitrary = (min, max) => Math.random() * (max - min) + min;
const getRandomInt = (min, max) => Math.floor(Math.random() * (max - min + 1)) + min;

class Solution {
  /**
   * @param {number[]} w
   */
  constructor(w) {
    for (let i = 1; i < w.length; i++) {
      w[i] += w[i - 1];
    }
    this.wSums = w;
  }

  /**
   * @return {number}
   */
  pickIndex() {
    const n = this.wSums.length;
    const idx = getRandomInt(1, this.wSums[n - 1]);

    let lo = 0;
    let hi = n - 1;

    while (lo < hi) {
      const mid = lo + Math.floor((hi - lo) / 2);

      if (this.wSums[mid] === idx) {
        return mid;
      } else if (this.wSums[mid] < idx) {
        lo = mid + 1;
      } else {
        hi = mid;
      }
    }

    return lo;
  }
}





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