java: add «2000. Reverse Prefix of Word»

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

java/reverse-prefix-of-word.java

@@ -0,0 +1,18 @@
+class Solution {
+  public String reversePrefix(String word, char ch) {
+    var sb = new StringBuilder(word.length());
+
+    var canSwap = true;
+    for (var i = 0; i < word.length(); ++i) {
+      char c = word.charAt(i);
+      sb.append(c);
+
+      if (c == ch && canSwap) {
+        sb.reverse();
+        canSwap = false;
+      }
+    }
+
+    return sb.toString();
+  }
+}