Add C++ solution for leetcode 390.

Author: yanglr

cpp-leetcode/leetcode390-elimination-game_find_law_recursive.cpp

@@ -0,0 +1,24 @@
+#include<algorithm>
+#include<vector>
+#include<iostream>
+using namespace std;
+
+class Solution {
+public:
+    int lastRemaining(int n) {
+        if (n == 1) return 1;
+        /* 递归: f(n) = 2(n/2 + 1 - f(n/2)) */
+        return n == 1 ? 1 : 2 * (n/2 + 1 - lastRemaining(n/2));
+    }
+};
+
+// Test
+int main()
+{
+    Solution sol;
+    int n = 4;
+    auto res = sol.lastRemaining(n);
+    cout << res << endl;
+
+    return 0;
+}