java: add «921. Minimum Add to Make Parentheses Valid»

URL:	https://leetcode.com/problems/minimum-add-to-make-parentheses-valid/
Signed-off-by: Matej Focko <me@mfocko.xyz>

java/minimum-add-to-make-parentheses-valid.java

@@ -0,0 +1,25 @@
+class Solution {
+  public int minAddToMakeValid(String s) {
+    int open = 0, needed = 0;
+
+    for (var c : s.toCharArray()) {
+      switch (c) {
+        case '(':
+          ++open;
+          break;
+        case ')':
+          if (open > 0) {
+            --open;
+          } else {
+            ++needed;
+          }
+          break;
+        default:
+          /* no-op */
+          break;
+      }
+    }
+
+    return open + needed;
+  }
+}