Random Pick with Weight

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

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% ).