Day-5 Random Pick with Weight

class Solution {
   
    int[] w=null;
    
    public Solution(int[] w) {
        for(int i=1;i<w.length;i++){
                w[i]+=w[i-1];
         }
        this.w=w;
    }
    
    public int pickIndex() { 
        double val = w[w.length-1] * Math.random();
        int left=0,right=w.length;
        while(left < right){
            int mid=left + (right-left)/2;
            if(w[mid]<val){
                left= mid+1;
            }
            else{
                right=mid;
            }
        }
       return left;
        
    }
}

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