Random Pick with Weight

class Solution {
    private int[] weight;
    private int[] dp;
    private int max;
    public Solution(int[] w) {
        weight = w;
        //Arrays.sort(weight);
        dp = new int[weight.length+1];
        for(int i = 0; i<weight.length; i++){
            dp[i+1] = dp[i] + weight[i];
            max+=weight[i];
        }
    }
    
    public int pickIndex() {
        int index = (int)(Math.random() * max);
        int res = 0;
        for(int i = 0; i<dp.length-1; i++){
            if(index >= dp[i] && index <= dp[i+1]){
                res = i;
            }
        }
        return res;
    }
}

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