# Day 5 of June LeetCode Challenge

Given an array `w`

of positive integers, where `w[i]`

describes the weight of index `i`

, write a function `pickIndex`

which randomly picks an index in proportion to its weight.

Note:

`1 <= w.length <= 10000`

`1 <= w[i] <= 10^5`

`pickIndex`

will be called at most`10000`

times.