Update count-of-range-sum.cpp

Author: kamyu104

C++/count-of-range-sum.cpp

@@ -42,3 +42,45 @@ class Solution {
         return count;
     }
 };
+
+// Divide and Conquer solution.
+class Solution2 {
+public:
+    int countRangeSum(vector<int>& nums, int lower, int upper) {
+        vector<long long> sums(nums.size() + 1);
+        for (int i = 0; i < nums.size(); ++i) {
+            sums[i + 1] = sums[i] + nums[i];
+        }
+        return countAndMergeSort(&sums, 0, sums.size() - 1, lower, upper);
+    }
+
+    int countAndMergeSort(vector<long long> *sums, int start, int end, int lower, int upper) {
+        if (end - start <= 0) {  // The number of range [start, end] of which size is less than 2 is always 0.
+            return 0;
+        }
+        int mid = start + (end - start) / 2;
+        int count = countAndMergeSort(sums, start, mid, lower, upper) +
+                    countAndMergeSort(sums, mid + 1, end, lower, upper);
+        int j = mid + 1, k = mid + 1, r = mid + 1;
+        vector<long long> tmp;
+        for (int i = start; i <= mid; ++i) {
+            // Count the number of range sums that lie in [lower, upper].
+            while (k <= end && (*sums)[k] - (*sums)[i] < lower) {
+                ++k;
+            }
+            while (j <= end && (*sums)[j] - (*sums)[i] <= upper) {
+                ++j;
+            }
+            count += j - k;
+
+            // Merge the two sorted arrays into tmp.
+            while (r <= end && (*sums)[r] < (*sums)[i]) {
+                tmp.emplace_back((*sums)[r++]);
+            }
+            tmp.emplace_back((*sums)[i]);
+        }
+        // Copy tmp back to sums.
+        copy(tmp.begin(), tmp.end(), sums->begin() + start);
+        return count;
+    }
+};