add cpp solutoin for maximum subarray

Author: jinshendan

30-Day-Leetcoding-Challenge/03-Maximum-Subarray/Maximum-Subarray.cpp

@@ -0,0 +1,12 @@
+class Solution {
+public:
+    int maxSubArray(vector<int>& nums) {
+        int sum = nums[0];
+        int ans = nums[0];
+        for (int i = 1; i < nums.size(); i++){
+            sum = max(sum + nums[i], nums[i]);
+            ans = max(ans, sum);
+        }
+        return ans;
+    }
+};