java: add «2109. Adding Spaces to a String»

URL:	https://leetcode.com/problems/adding-spaces-to-a-string/
Signed-off-by: Matej Focko <me@mfocko.xyz>

java/adding-space-to-a-string.java

@@ -0,0 +1,18 @@
+class Solution {
+  public String addSpaces(String s, int[] spaces) {
+    var result = new StringBuilder();
+    result.ensureCapacity(s.length() + spaces.length);
+
+    int i = 0;
+    for (int j = 0; j < s.length(); ++j) {
+      if (i < spaces.length && spaces[i] == j) {
+        result.append(' ');
+        ++i;
+      }
+
+      result.append(s.charAt(j));
+    }
+
+    return result.toString();
+  }
+}