LeetCode - Random Pick with Weight

You are given an array of positive integers w where w[i] describes the weight of ith index (0-indexed).

We need to call the function pickIndex() which randomly returns an integer in the range [0, w.length - 1]. pickIndex() should return the integer proportional to its weight in the w array. For example, for w = [1, 3], the probability of picking the index 0 is 1 / (1 + 3) = 0.25 (i.e 25%) while the probability of picking the index 1 is 3 / (1 + 3) = 0.75 (i.e 75%).

More formally, the probability of picking index i is w[i] / sum(w).

 

Example 1:

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

Explanation
Solution solution = new Solution([1]);
solution.pickIndex(); // return 0. Since there is only one single element on the array the only option is to return the first element.
Example 2:

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

Explanation
Solution solution = new Solution([1, 3]);
solution.pickIndex(); // return 1. It's returning the second element (index = 1) that has probability of 3/4.
solution.pickIndex(); // return 1
solution.pickIndex(); // return 1
solution.pickIndex(); // return 1
solution.pickIndex(); // return 0. It's returning the first element (index = 0) that has probability of 1/4.

Since this is a randomization problem, multiple answers are allowed so the following outputs can be considered correct :
[null,1,1,1,1,0]
[null,1,1,1,1,1]
[null,1,1,1,0,0]
[null,1,1,1,0,1]
[null,1,0,1,0,0]
......
and so on.

class Solution {
    private int[] arr;
    private Random rand;
    public Solution(int[] w) {
        rand = new Random();
        arr = new int[w.length + 1];
        arr[0] = 0;
        int cur = 0;
        for (int i = 1; i <= w.length; i++) {
            arr[i] = cur + w[i-1];
            cur = arr[i];
        }
    }
    
    public int pickIndex() {
        int randInt = rand.nextInt(arr[arr.length-1]) + 1;
        // TODO Improve
        for(int i = 1; i < arr.length; i++) {
            if (randInt <= arr[i]) {
                return i-1;
            }
        }
        return arr.length-2;
    }
}

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

标签：index,return,Weight,Random,pickIndex,Solution,solution,null,LeetCode
来源： https://www.cnblogs.com/incrediblechangshuo/p/14425584.html