Add K Sum Subarrays

Author: marttp

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
+}