[LC] 528. 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.

Note:

1. 1 <= w.length <= 10000
2. 1 <= w[i] <= 10^5
3. pickIndex will be called at most 10000 times.

Example 1:

Input: 
["Solution","pickIndex"]
[[[1]],[]]
Output: [null,0]

Example 2:

Input: 
["Solution","pickIndex","pickIndex","pickIndex","pickIndex","pickIndex"]
[[[1,3]],[],[],[],[],[]]
Output: [null,0,1,1,1,0]

Explanation of Input Syntax:

The input is two lists: the subroutines called and their arguments. Solution's constructor has one argument, the array w. pickIndex has no arguments. Arguments are always wrapped with a list, even if there aren't any.

class Solution {
    int[] arr;
    private Random rand;

    public Solution(int[] w) {
        arr = new int[w.length];
        arr[0] = w[0];
        for (int i = 1; i < w.length; i++) {
            arr[i] = arr[i - 1] + w[i];
        }
    }
    
    public int pickIndex() {
        rand = new Random();
        int num = rand.nextInt(arr[arr.length - 1]) + 1;
        int left = 0, right = arr.length - 1;
        while (left <= right) {
            int mid = left + (right - left) / 2;
            if (arr[mid] == num) {
                return mid;
            } else if (arr[mid] < num) {
                left = mid + 1;
            } else {
                right = mid - 1;
            }
        }
        return left;
    }
}

/**
 * Your Solution object will be instantiated and called as such:
 * Solution obj = new Solution(w);
 * int param_1 = obj.pickIndex();
 */