Given an array of positive integers w, implement pickIndex() that randomly picks an index in proportion to its weight.
Example 1:
Input: w = [1, 3]
pickIndex() // returns 0 with probability 1/4, 1 with probability 3/4
Example 2:
Input: w = [1, 1, 1]
pickIndex() // returns 0, 1, or 2 each with probability 1/3