Added task 20.

Author: javadev

src/main/java/s0020.valid.parentheses/Solution.java

@@ -0,0 +1,22 @@
+package s0020.valid.parentheses;
+
+import java.util.Stack;
+
+public class Solution {
+    public boolean isValid(String s) {
+        Stack<Character> stack = new Stack<Character>();
+        for (int i = 0; i < s.length(); i++) {
+            char c = s.charAt(i);
+            if (c == '(' || c == '[' || c == '{') {
+                stack.push(c);
+            } else if (c == ')' && !stack.isEmpty() && stack.peek() == '(') {
+                stack.pop();
+            } else if (c == '}' && !stack.isEmpty() && stack.peek() == '{') {
+                stack.pop();
+            } else if (c == ']' && !stack.isEmpty() && stack.peek() == '[') {
+                stack.pop();
+            } else return false;
+        }
+        return stack.isEmpty();
+    }
+}

src/test/java/s0020.valid.parentheses/SolutionTest.java

@@ -0,0 +1,13 @@
+package s0020.valid.parentheses;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import org.junit.Test;
+
+public class SolutionTest {
+    @Test
+    public void isValid() {
+        assertThat(new Solution().isValid("()"), equalTo(true));
+    }
+}