Random Pick By Weight - Day 5

Author: suriyakrishna

src/main/scala/com/kishan/scala/leetcode/juneChallenges/RandomPickWithWeight.scala

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+package com.kishan.scala.leetcode.juneChallenges
+
+import scala.util.Random
+
+
+/*
+* Given an array w of positive integers,
+* where w[i] describes the weight of index i,
+* write a function pickIndex which randomly picks an index in proportion to its weight.
+*
+*/
+
+/*
+* Input:
+  ["Solution","pickIndex"]
+  [[[1]],[]]
+  Output: [null,0]
+*
+* Input:
+  ["Solution","pickIndex","pickIndex","pickIndex","pickIndex","pickIndex"]
+  [[[1,3]],[],[],[],[],[]]
+  Output: [null,0,1,1,1,0]
+*/
+ */
+
+
+/*
+* Approach:
+*
+* 1. Calculation Running total for each element of the array.
+* 2. Generating a Random number using scala.util.Random (Including upper boundary Value).
+* 3. Using Binary Search Algorithm to search the range where the random number is located.
+* 4. Finally returning the upper boundary value of the range. Which is the result.
+*
+*  */
+
+/*
+* If Input: [8, 9]
+*
+* Running sum will be [8, 17]
+*
+* If our Random number lies between 1-8 we will pick the first index.
+* If our random number lies between 9-17 we will pick the second index
+*
+* We will ending up picking the index which has the highest probability
+*
+* */
+
+
+class RandomPickWithWeight(_w: Array[Int]) {
+
+  private val length = _w.length
+  private val totalSum = _w.sum
+  private val random = new Random()
+  private var preSum: Array[Int] = _w.scan(0)((a, b) => a + b).drop(1)
+
+  def pickIndex(): Int = {
+    val randomNumber = random.nextInt(totalSum) + 1
+    var head = 0
+    var tail = preSum.length - 1
+    var mid: Int = head + (tail - head) / 2
+    while (head < tail) {
+      mid = head + (tail - mid) / 2
+      if (randomNumber > preSum(mid)) {
+        head = mid + 1
+      } else {
+        tail = mid
+      }
+    }
+    return if (preSum(tail) >= randomNumber) tail else head
+  }
+
+
+}

src/main/scala/com/kishan/scala/leetcode/juneChallenges/RandomPickWithWeightTest.scala

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+package com.kishan.scala.leetcode.juneChallenges
+
+object RandomPickWithWeightTest {
+  def main(args: Array[String]): Unit = {
+    val input: Array[String] = Array("Solution", "pickIndex")
+    val inputInt: Array[Int] = input.map(_.length)
+    val r = new RandomPickWithWeight(inputInt)
+    println(r.pickIndex())
+  }
+}