Simplify condition check

Author: AndrewNikolin

Diff for: java/0020-valid-parentheses.java

@@ -8,15 +8,13 @@ public boolean isValid(String s) {
                 stack.isEmpty() &&
                 (s.charAt(i) == ')' || s.charAt(i) == '}' || s.charAt(i) == ']')
             ) return false; else {
-                if (!stack.isEmpty()) {
                     if (
-                        stack.peek() == '(' && s.charAt(i) == ')'
+                        s.charAt(i) == ')' && stack.peek() == '('
                     ) stack.pop(); else if (
-                        stack.peek() == '{' && s.charAt(i) == '}'
+                        s.charAt(i) == '}' && stack.peek() == '{'
                     ) stack.pop(); else if (
-                        stack.peek() == '[' && s.charAt(i) == ']'
+                        s.charAt(i) == ']' && stack.peek() == '['
                     ) stack.pop(); else stack.add(s.charAt(i));
-                } else stack.add(s.charAt(i));
             }
         }
         return stack.isEmpty();