Merge pull request ByteByteGoHq#16 from marttp/kotlin-prefix-sums

Kotlin Chapter 10: Prefix Sums

Author: Destiny-02

kotlin/Prefix Sums/KSumSubarrays.kt

@@ -0,0 +1,19 @@
+fun kSumSubarrays(nums: List<Int>, k: Int): Int {
+    val n = nums.size
+    var count = 0
+    // Populate the prefix sum array, setting its first element to 0.
+    val prefixSum = IntArray(n + 1)
+    for (i in 0 until n) {
+        prefixSum[i + 1] = prefixSum[i] + nums[i]
+    }
+    // Loop through all valid pairs of prefix sum values to find all 
+    // subarrays that sum to 'k'.
+    for (j in 1..n) {
+        for (i in 1..j) {
+            if (prefixSum[j] - prefixSum[i - 1] == k) {
+                count++
+            }
+        }
+    }
+    return count
+}

kotlin/Prefix Sums/KSumSubarraysOptimized.kt

@@ -0,0 +1,20 @@
+fun kSumSubarraysOptimized(nums: IntArray, k: Int): Int {
+    var count = 0
+    // Initialize the map with 0 to handle subarrays that sum to 'k'
+    // from the start of the array.
+    val prefixSumMap = mutableMapOf(0 to 1)
+    var currPrefixSum = 0
+    for (num in nums) {
+        // Update the running prefix sum by adding the current number.
+        currPrefixSum += num
+        // If a subarray with sum 'k' exists, increment 'count' by the
+        // number of times it has been found.
+        if (currPrefixSum - k in prefixSumMap) {
+            count += prefixSumMap[currPrefixSum - k]!!
+        }
+        // Update the frequency of 'currPrefixSum' in the hash map.
+        val freq = prefixSumMap.getOrDefault(currPrefixSum, 0)
+        prefixSumMap[currPrefixSum] = freq + 1
+    }
+    return count
+}

kotlin/Prefix Sums/ProductsArrayWithoutCurrentElement.kt

@@ -0,0 +1,16 @@
+fun productArrayWithoutCurrentElement(nums: List<Int>): List<Int> {
+    val n = nums.size
+    val res = MutableList(n) { 1 }
+    // Populate the output with the running left product.
+    for (i in 1 until n) {
+        res[i] = res[i - 1] * nums[i - 1]
+    }
+    // Multiply the output with the running right product, from right to 
+    // left.
+    var rightProduct = 1
+    for (i in n - 1 downTo 0) {
+        res[i] *= rightProduct
+        rightProduct *= nums[i]
+    }
+    return res
+}

kotlin/Prefix Sums/SumBetweenRange.kt

@@ -0,0 +1,18 @@
+class SumBetweenRange(nums: IntArray) {
+
+    private val prefixSum: IntArray = IntArray(nums.size)
+
+    init {
+        prefixSum[0] = nums[0]
+        for (i in 1 until nums.size) {
+            prefixSum[i] = prefixSum[i - 1] + nums[i]
+        }
+    }
+
+    fun sumRange(i: Int, j: Int): Int {
+        if (i == 0) {
+            prefixSum[j]
+        }
+        return prefixSum[j] - prefixSum[i - 1]
+    }
+}