problems(cpp): add “20. Valid Parentheses”

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

problems/valid-parentheses.cpp

@@ -0,0 +1,32 @@
+#include <string>
+#include <vector>
+
+class Solution {
+public:
+    bool isValid(const std::string& s)
+    {
+        std::vector<char> st;
+
+        for (auto c : s) {
+            switch (c) {
+            case '(':
+                st.push_back(')');
+                break;
+            case '{':
+                st.push_back('}');
+                break;
+            case '[':
+                st.push_back(']');
+                break;
+            default:
+                if (st.empty() || st.back() != c) {
+                    return false;
+                }
+                st.pop_back();
+                break;
+            }
+        }
+
+        return st.empty();
+    }
+};