problems(cpp): add “1903. Largest Odd Number in String”

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/cpp/largest-odd-number-in-string.cpp

@@ -0,0 +1,13 @@
+#include <string>
+
+class Solution {
+public:
+    std::string largestOddNumber(const std::string& num) {
+        auto i = num.find_last_of("13579");
+        if (i == std::string::npos) {
+            return "";
+        }
+
+        return num.substr(0, i + 1);
+    }
+};