Random Pick with Weight

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.

 class Solution {
     Random random;
     int[] wSums;

     public Solution(int[] w) {
         this.random = new Random();
         for (int i = ; i < w.length; ++i) {
             w[i] += w[i - ];
         }
         this.wSums = w;
     }

     public int pickIndex() {
         int len = wSums.length;
         int idx = random.nextInt(wSums[len - ]) + ;
         int left = , right = len - ;
         while (left <= right) {
             int mid = left + (right - left) / ;
             if (wSums[mid] == idx) {
                 return mid;
             } else if (wSums[mid] < idx) {
                 left = mid + ;
             } else {
                 right = mid - ;
             }
         }
         return left;
     }
 }