java: add «32. Longest Valid Parentheses»

Signed-off-by: Matej Focko <me@mfocko.xyz>

java/longest-valid-parentheses.java

@@ -0,0 +1,24 @@
+import java.util.Stack;
+
+class Solution {
+  public int longestValidParentheses(String s) {
+    var longest = 0;
+    var indices = new Stack<Integer>();
+
+    indices.push(-1);
+    for (var i = 0; i < s.length(); ++i) {
+      if (s.charAt(i) == '(') {
+        indices.push(i);
+      } else {
+        indices.pop();
+        if (indices.isEmpty()) {
+          indices.push(i);
+        } else {
+          longest = Math.max(longest, i - indices.peek());
+        }
+      }
+    }
+
+    return longest;
+  }
+}