java: add «394. Decode String»

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

java/decode-string.java

@@ -0,0 +1,35 @@
+import java.util.Stack;
+
+class Solution {
+  private record StackEntry(int count, StringBuilder str) {
+    @Override
+    public String toString() {
+      return new StringBuilder().repeat(str(), count()).toString();
+    }
+  }
+
+  public String decodeString(String s) {
+    var stack = new Stack<StackEntry>();
+    stack.push(new StackEntry(1, new StringBuilder()));
+
+    int count = 0;
+    for (int i = 0; i < s.length(); ++i) {
+      var c = s.charAt(i);
+
+      if (c == '[') {
+        stack.push(new StackEntry(count, new StringBuilder()));
+        count = 0;
+      } else if (c == ']') {
+        var last = stack.pop();
+        stack.peek().str().append(last);
+      } else if (Character.isDigit(c)) {
+        count *= 10;
+        count += c - '0';
+      } else {
+        stack.peek().str().append(c);
+      }
+    }
+
+    return stack.pop().toString();
+  }
+}