feat: add cpp solution to lcof problem: No.51

Author: maolonglong

Diff for: lcof/面试题51. 数组中的逆序对/README.md

@@ -157,6 +157,41 @@ var reversePairs = function (nums) {
 };
 ```
 
+### **C++**
+
+```cpp
+class Solution {
+public:
+    int reversePairs(vector<int>& nums) {
+        int n = nums.size();
+        vector<int> temp(n);
+        return mergeSort(nums, temp, 0, n - 1);
+    }
+
+private:
+    int mergeSort(vector<int>& nums, vector<int>& temp, int l, int r) {
+        if (l >= r) {
+            return 0;
+        }
+        int m = l + (r - l) / 2;
+        int count = mergeSort(nums, temp, l, m) + mergeSort(nums, temp, m + 1, r);
+        int i = l, j = m + 1, k = l;
+        while (i <= m || j <= r) {
+            if (i > m) {
+                temp[k++] = nums[j++];
+            } else if (j > r || nums[i] <= nums[j]) {
+                temp[k++] = nums[i++];
+            } else {
+                count += m - i + 1;
+                temp[k++] = nums[j++];
+            }
+        }
+        copy(temp.begin() + l, temp.begin() + r + 1, nums.begin() + l);
+        return count;
+    }
+};
+```
+
 ### **...**
 
 ```

Diff for: lcof/面试题51. 数组中的逆序对/Solution.cpp

@@ -0,0 +1,30 @@
+class Solution {
+public:
+    int reversePairs(vector<int>& nums) {
+        int n = nums.size();
+        vector<int> temp(n);
+        return mergeSort(nums, temp, 0, n - 1);
+    }
+
+private:
+    int mergeSort(vector<int>& nums, vector<int>& temp, int l, int r) {
+        if (l >= r) {
+            return 0;
+        }
+        int m = l + (r - l) / 2;
+        int count = mergeSort(nums, temp, l, m) + mergeSort(nums, temp, m + 1, r);
+        int i = l, j = m + 1, k = l;
+        while (i <= m || j <= r) {
+            if (i > m) {
+                temp[k++] = nums[j++];
+            } else if (j > r || nums[i] <= nums[j]) {
+                temp[k++] = nums[i++];
+            } else {
+                count += m - i + 1;
+                temp[k++] = nums[j++];
+            }
+        }
+        copy(temp.begin() + l, temp.begin() + r + 1, nums.begin() + l);
+        return count;
+    }
+};