From 4aea9f71ebc0effdb9db5aa40cbee6d82384033c Mon Sep 17 00:00:00 2001
From: Jer3myYu <jeremyyucareer@gmail.com>
Date: Wed, 29 Jan 2025 23:33:43 -0500
Subject: [PATCH 1/3] add: SumBetweenRange

---
 java/Prefix Sums/SumBetweenRange.java | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)
 create mode 100644 java/Prefix Sums/SumBetweenRange.java

diff --git a/java/Prefix Sums/SumBetweenRange.java b/java/Prefix Sums/SumBetweenRange.java
new file mode 100644
index 0000000..b1b08fc
--- /dev/null
+++ b/java/Prefix Sums/SumBetweenRange.java	
@@ -0,0 +1,18 @@
+public class SumBetweenRange {
+    int[] prefixSum;
+
+    public SumBetweenRange(int[] nums) {
+        this.prefixSum = new int[nums.length];
+        this.prefixSum[0] = nums[0];
+        for (int i = 1; i < nums.length; i++) {
+            this.prefixSum[i] = this.prefixSum[i - 1] + nums[i];
+        }
+    }
+
+    public int sumRange(int i, int j) {
+        if (i == 0) {
+            return this.prefixSum[j];
+        }
+        return this.prefixSum[j] - this.prefixSum[i - 1];
+    }
+}

From e0d39ecca9e3982487e223b8e99c0211174a99e1 Mon Sep 17 00:00:00 2001
From: Jer3myYu <jeremyyucareer@gmail.com>
Date: Thu, 30 Jan 2025 00:10:52 -0500
Subject: [PATCH 2/3] add: KSumSubarrays and KSumSubarraysOptimized

---
 java/Prefix Sums/KSumSubarrays.java          | 21 +++++++++++++++
 java/Prefix Sums/KSumSubarraysOptimized.java | 27 ++++++++++++++++++++
 2 files changed, 48 insertions(+)
 create mode 100644 java/Prefix Sums/KSumSubarrays.java
 create mode 100644 java/Prefix Sums/KSumSubarraysOptimized.java

diff --git a/java/Prefix Sums/KSumSubarrays.java b/java/Prefix Sums/KSumSubarrays.java
new file mode 100644
index 0000000..a7ae278
--- /dev/null
+++ b/java/Prefix Sums/KSumSubarrays.java	
@@ -0,0 +1,21 @@
+public class KSumSubarrays {
+    public int kSumSubarrays(int[] nums, int k) {
+        int n = nums.length;
+        int count = 0;
+        // Populate the prefix sum array, setting its first element to 0.
+        int[] prefixSum = new int[nums.length + 1];
+        for (int i = 0; i < n; i++) {
+            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 (int j = 1; j < n + 1; j++) {
+            for (int i = 1; i < j + 1; i++) {
+                if (prefixSum[j] - prefixSum[i - 1] == k) {
+                    count++;
+                }
+            }
+        }
+        return count;
+    }
+}
diff --git a/java/Prefix Sums/KSumSubarraysOptimized.java b/java/Prefix Sums/KSumSubarraysOptimized.java
new file mode 100644
index 0000000..a85f543
--- /dev/null
+++ b/java/Prefix Sums/KSumSubarraysOptimized.java	
@@ -0,0 +1,27 @@
+import java.util.HashMap;
+import java.util.Map;
+
+public class KSumSubarraysOptimized {
+    public int kSumSubarraysOptimized(int[] nums, int k) {
+        int count = 0;
+        // Initialize the map with 0 to handle subarrays that sum to 'k' 
+        // from the start of the array.
+        Map<Integer, Integer> prefixSumMap = new HashMap<>();
+        prefixSumMap.put(0, 1);
+        int currPrefixSum = 0;
+        for (int num : 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 (prefixSumMap.containsKey(currPrefixSum - k)) {
+                count += prefixSumMap.get(currPrefixSum - k);
+            }
+            // Update the frequency of 'curr_prefix_sum' in the hash map.
+            int freq = prefixSumMap.getOrDefault(currPrefixSum, 0);
+            prefixSumMap.put(currPrefixSum, freq + 1);
+        }
+        return count;
+    }
+
+}

From 88f6f8907272804ee106f405ab56ad2b2b4bd844 Mon Sep 17 00:00:00 2001
From: Jer3myYu <jeremyyucareer@gmail.com>
Date: Thu, 30 Jan 2025 00:11:05 -0500
Subject: [PATCH 3/3] add: ProductArrayWithoutCurrentElement

---
 .../ProductArrayWithoutCurrentElement.java    | 21 +++++++++++++++++++
 1 file changed, 21 insertions(+)
 create mode 100644 java/Prefix Sums/ProductArrayWithoutCurrentElement.java

diff --git a/java/Prefix Sums/ProductArrayWithoutCurrentElement.java b/java/Prefix Sums/ProductArrayWithoutCurrentElement.java
new file mode 100644
index 0000000..296df01
--- /dev/null
+++ b/java/Prefix Sums/ProductArrayWithoutCurrentElement.java	
@@ -0,0 +1,21 @@
+import java.util.Arrays;
+
+public class ProductArrayWithoutCurrentElement {
+    public int[] productArrayWithoutCurrentElement(int[] nums) {
+        int n = nums.length;
+        int[] res = new int[n];
+        Arrays.fill(res, 1);
+        // Populate the output with the running left product.
+        for (int i = 1; i < n; i++) {
+            res[i] = res[i - 1] * nums[i - 1];
+        }
+        // Multiply the output with the running right product, from right to 
+        // left.
+        int rightProduct = 1;
+        for (int i = n - 1; i > -1; i--) {
+            res[i] *= rightProduct;
+            rightProduct *= nums[i];
+        }
+        return res;
+    }
+}