upload

2/E.cpp

@@ -0,0 +1,79 @@
+#include <iostream>
+
+void Merge(int* arr, int left, int mid, int right, long long& counter);
+void MergeSort(int* arr, int start, int end, long long& counter);
+
+void Merge(int* arr, int left, int mid, int right, long long& counter) {
+  int left_size = mid-left+1;
+  int right_size = right-mid;
+
+  int* left_arr = new int[left_size];
+  int* right_arr = new int[right_size];
+
+  for (int i = 0; i < left_size; ++i) {
+    left_arr[i] = arr[left + i];
+  }
+
+  for (int i = 0; i < right_size; ++i) {
+    right_arr[i] = arr[mid + 1 + i];
+  }
+
+  int i = 0, j = 0, idx = left;
+
+  while (i < left_size && j < right_size) {
+    if (left_arr[i] <= right_arr[j]) {
+      arr[idx] = left_arr[i];
+      ++i;
+    } else {
+      arr[idx] = right_arr[j];
+      ++j;
+      counter += left_size - i;
+    }
+    ++idx;
+  }
+
+  while (i < left_size) {
+    arr[idx] = left_arr[i];
+    ++i;
+    ++idx;
+  }
+
+  while (j < right_size) {
+    arr[idx] = right_arr[j];
+    ++j;
+    ++idx;
+  }
+
+  delete[] left_arr;
+  delete[] right_arr;
+}
+
+void MergeSort(int* arr, int start, int end, long long& counter) {
+  if (start >= end)
+    return;
+
+  int mid = start + (end - start) / 2;
+  MergeSort(arr, start, mid, counter);
+  MergeSort(arr, mid + 1, end, counter);
+
+  Merge(arr, start, mid, end, counter);
+}
+
+int main() {
+  int size;
+  std::cin >> size;
+  int* a = new int[size];
+
+  for (int i = 0; i < size; ++i) {
+    std::cin >> a[i];
+  }
+
+  long long counter = 0;
+
+  MergeSort(a, 0, size - 1, counter);
+
+  std::cout << counter;
+
+  delete[] a;
+  return 0;
+}