add a more concise solution for 20

Author: fishercoder1534

src/main/java/com/fishercoder/solutions/_20.java

@@ -59,4 +59,26 @@ public boolean isValid(String s) {
             return stack.isEmpty();
         }
     }
+
+    public static class Solution2 {
+        /**
+         * A more concise solution:
+         * credit: https://leetcode.com/problems/valid-parentheses/discuss/9178/Short-java-solution
+         * */
+        public boolean isValid(String s) {
+            Stack<Character> stack = new Stack<>();
+            for (char c : s.toCharArray()) {
+                if (c == '(') {
+                    stack.push(')');
+                } else if (c == '{') {
+                    stack.push('}');
+                } else if (c == '[') {
+                    stack.push(']');
+                } else if (stack.isEmpty() || stack.pop() != c) {
+                    return false;
+                }
+            }
+            return stack.isEmpty();
+        }
+    }
 }