LeetCode in Kotlin

528. Random Pick with Weight

Medium

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”] [[[1]],[]]

Output: [null,0]

Explanation:

Solution solution = new Solution([1]); 
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
• pickIndex will be called at most 104 times.

Solution

import java.util.Random
import java.util.TreeMap

@Suppress("kotlin:S2245")
class Solution(val w: IntArray) {

    var x: IntArray = IntArray(w.size) { 0 }
    val rand = Random()
    val tree = TreeMap<Int, Int>()
    var sum = 0

    init {
        for (i in w.indices) {
            tree.put(sum, i)
            sum += w[i]
        }
    }

    fun pickIndex(): Int {
        val r = rand.nextInt(sum)
        return tree.floorEntry(r).value!!
    }
}

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

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