feat: add solutions to lc/lcof2 problems

lcof2/剑指 Offer II 012. 左右两边子数组的和相等/README.md

@@ -122,6 +122,28 @@ func pivotIndex(nums []int) int {
 }
 ```
 
+### **C++**
+
+```cpp
+class Solution {
+public:
+    int pivotIndex(vector<int>& nums) {
+        int sum = 0;
+        int total = 0;
+        for (int num: nums)
+            sum += num;
+
+        for (int i = 0; i < nums.size(); i++) {
+            total += nums[i];
+            if (total - nums[i] == sum - total)
+                return i;
+        }
+
+        return -1;
+    }
+};
+```
+
 ### **...**
 
 ```

lcof2/剑指 Offer II 012. 左右两边子数组的和相等/Solution.cpp

@@ -0,0 +1,17 @@
+class Solution {
+public:
+    int pivotIndex(vector<int>& nums) {
+        int sum = 0;
+        int total = 0;
+        for (int num: nums)
+            sum += num;
+
+        for (int i = 0; i < nums.size(); i++) {
+            total += nums[i];
+            if (total - nums[i] == sum - total)
+                return i;
+        }
+
+        return -1;
+    }
+};

solution/1900-1999/1991.Find the Middle Index in Array/README.md

@@ -67,6 +67,28 @@
 
 <!-- tabs:start -->
 
+### **C++**
+
+```cpp
+class Solution {
+public:
+    int findMiddleIndex(vector<int>& nums) {
+        int sum = 0;
+        int total = 0;
+        for(int num: nums)
+            sum += num;
+
+        for(int i = 0; i < nums.size(); i++) {
+            total += nums[i];
+            if(total - nums[i] == sum - total)
+                return i;
+        }
+
+        return -1;
+    }
+};
+```
+
 ### **Python3**
 
 <!-- 这里可写当前语言的特殊实现逻辑 -->

solution/1900-1999/1991.Find the Middle Index in Array/README_EN.md

@@ -65,6 +65,28 @@ The sum of the numbers after index 0 is: 0
 
 <!-- tabs:start -->
 
+### **C++**
+
+```cpp
+class Solution {
+public:
+    int findMiddleIndex(vector<int>& nums) {
+        int sum = 0;
+        int total = 0;
+        for(int num: nums)
+            sum += num;
+
+        for(int i = 0; i < nums.size(); i++) {
+            total += nums[i];
+            if(total - nums[i] == sum - total)
+                return i;
+        }
+
+        return -1;
+    }
+};
+```
+
 ### **Python3**
 
 ```python

solution/1900-1999/1991.Find the Middle Index in Array/Solution.cpp

@@ -0,0 +1,17 @@
+class Solution {
+public:
+    int findMiddleIndex(vector<int>& nums) {
+        int sum = 0;
+        int total = 0;
+        for(int num: nums)
+            sum += num;
+
+        for(int i = 0; i < nums.size(); i++) {
+            total += nums[i];
+            if(total - nums[i] == sum - total)
+                return i;
+        }
+
+        return -1;
+    }
+};