You are given a 0-indexed array of positive integers w where w[i] describes the weight of the ith index.

Implement the Solution class:

• Solution(int[] w) — initializes the object with the array w.
• int pickIndex() — returns a random index, weighted proportionally to w[i].

Example:

w = [1, 3] → pickIndex returns 0 with 25% probability, 1 with 75% probability.