Merge pull request ByteByteGoHq#34 from ongshunping/cpp-solutions-prefix-sums

Add C++ solutions for Chapter 10 (Prefix Sums)

Author: Destiny-02

cpp/Prefix Sums/k_sum_subarrays.cpp

@@ -0,0 +1,22 @@
+#include <vector>
+
+int kSumSubarrays(std::vector<int>& nums, int k) {
+    int n = nums.size();
+    int count = 0;
+    // Populate the prefix sum array, setting its first element to 0.
+    std::vector<int> prefixSum;
+    prefixSum.push_back(0);
+    for (int i = 0; i < n; i++) {
+        prefixSum.push_back(prefixSum.back() + 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; j++) {
+        for (int i = 1; i <= j; i++) {
+            if (prefixSum[j] - prefixSum[i - 1] == k) {
+                count += 1;
+            }
+        }
+    }
+    return count;
+}

cpp/Prefix Sums/k_sum_subarrays_optimized.cpp

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+#include <vector>
+#include <unordered_map>
+
+int kSumSubarraysOptimized(std::vector<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.
+    std::unordered_map<int, int> prefixSumMap;
+    prefixSumMap[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.find(currPrefixSum - k) != prefixSumMap.end()) {
+            count += prefixSumMap[currPrefixSum - k];
+        }
+        // Update the frequency of 'currPrefixSum' in the hash map.
+        int freq = prefixSumMap[currPrefixSum];
+        prefixSumMap[currPrefixSum] = freq + 1;
+    }
+    return count;
+}

cpp/Prefix Sums/product_array_without_current_element.cpp

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+#include <vector>
+
+std::vector<int> productArrayWithoutCurrentElement(std::vector<int>& nums) {
+    int n = nums.size();
+    std::vector<int> res(n, 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 >= 0; i--) {
+        res[i] *= rightProduct;
+        rightProduct *= nums[i];
+    }
+    return res;
+}

cpp/Prefix Sums/sum_between_range.cpp

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+#include <vector>
+
+class SumBetweenRange {
+public:
+    SumBetweenRange(std::vector<int>& nums) {
+        if (!nums.empty()) {
+            prefixSum.push_back(nums[0]);
+            for (int i = 1; i < nums.size(); i++) {
+                prefixSum.push_back(prefixSum.back() + nums[i]);
+            }
+        }
+    }
+
+    int sumRange(int i, int j) {
+        if (i == 0) {
+            return prefixSum[j];
+        }
+        return prefixSum[j] - prefixSum[i - 1];
+    }
+
+private:
+    std::vector<int> prefixSum;
+};