java: add «2938. Separate Black and White Balls»

URL:	https://leetcode.com/problems/separate-black-and-white-balls/
Signed-off-by: Matej Focko <me@mfocko.xyz>

java/separate-black-and-white-balls.java

@@ -0,0 +1,19 @@
+class Solution {
+  public long minimumSteps(String s) {
+    long swaps = 0;
+
+    int blackBalls = 0;
+    for (int i = 0; i < s.length(); ++i) {
+      switch (s.charAt(i)) {
+        case '0':
+          swaps += blackBalls;
+          break;
+        case '1':
+          ++blackBalls;
+          break;
+      }
+    }
+
+    return swaps;
+  }
+}