feature: add 2874 solution1 for C++

Author: PerfectPan

solution/2874/sol1.cpp

@@ -0,0 +1,17 @@
+class Solution {
+public:
+    long long maximumTripletValue(vector<int>& nums) {
+        long long ans = 0;
+        int n = nums.size();
+        vector<int> prefix(n + 1, 0);
+        for (int i = 0; i < n; ++i) {
+            prefix[i + 1] = max(prefix[i], nums[i]);
+        }
+        int mx = nums[n - 1];
+        for (int i = n - 2; i >= 1; --i) {
+            ans = max(ans, 1LL * (prefix[i] - nums[i]) * mx);
+            mx = max(mx, nums[i]);
+        }
+        return ans;
+    }
+};