# 528. Random Pick with Weight LeetCode Solution | Easy Approach Share:

Random Pick with Weight You are given a 0-indexed array of positive integers `w` where `w[i]` describes the weight of the `ith` index.

You need to implement the function `pickIndex()`, which randomly picks an index in the range `[0, w.length - 1]` (inclusive) and returns it. The probability of picking an index `i` is `w[i] / sum(w)`.

• For example, if `w = [1, 3]`, the probability of picking index `0` is `1 / (1 + 3) = 0.25` (i.e., `25%`), and the probability of picking index `1` is `3 / (1 + 3) = 0.75` (i.e., `75%`).

Example 1:

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

Explanation Solution solution = new Solution(); solution.pickIndex(); // return 0. The only option is to return 0 since there is only one element in w.

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 is returning the second element (index = 1) that has a probability of 3/4. solution.pickIndex(); // return 1 solution.pickIndex(); // return 1 solution.pickIndex(); // return 1 solution.pickIndex(); // return 0. It is returning the first element (index = 0) that has a probability of 1/4. Since this is a randomization problem, multiple answers are allowed. All of 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.

Constraints:

• `1 <= w.length <= 104`
• `1 <= w[i] <= 105`

Time: O(n)
Space: O(n)

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