Update 0020-valid-parentheses.java

Added additional solution which implements a HashMap to store a lookup table of brackets to check the solution.

java/0020-valid-parentheses.java

@@ -22,3 +22,31 @@ public boolean isValid(String s) {
         return stack.isEmpty();
     }
 }
+
+//Solution with HashMap Lookup table as described in the video
+
+class Solution {
+    public boolean isValid(String s) {
+        Stack<Character> brackets = new Stack<>();
+        Map<Character, Character> bracketLookup = new HashMap<>();
+
+        bracketLookup.put(')', '(');
+        bracketLookup.put('}', '{');
+        bracketLookup.put(']', '[');
+
+        for (char c : s.toCharArray()) {
+            if (bracketLookup.containsKey(c)) {
+                if (brackets.size() != 0 && brackets.peek() == bracketLookup.get(c)) {
+                    brackets.pop();
+                } else {
+                    return false;
+                }
+            } else {
+                brackets.push(c);
+            }
+        }
+
+        if (brackets.size() == 0) return true;
+        return false;
+    }
+}