kt: add «1980. Find Unique Binary String»

URL:	https://leetcode.com/problems/find-unique-binary-string/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/find-unique-binary-string.kt

@@ -0,0 +1,12 @@
+class Solution {
+    companion object {
+        private val MAPPING: Map<Char, Char> = mapOf('0' to '1', '1' to '0')
+    }
+
+    fun findDifferentBinaryString(nums: Array<String>): String =
+        nums
+            .withIndex()
+            .fold(StringBuilder(nums.size)) { sb, it ->
+                sb.append(MAPPING[it.value[it.index]]!!)
+            }.toString()
+}