Java Chapter 10: Prefix Sums

java/Prefix Sums/KSumSubarrays.java

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+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;
+    }
+}

java/Prefix Sums/KSumSubarraysOptimized.java

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+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;
+    }
+
+}

java/Prefix Sums/ProductArrayWithoutCurrentElement.java

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+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;
+    }
+}

java/Prefix Sums/SumBetweenRange.java

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+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];
+    }
+}