kt: add «14. Longest Common Prefix»

URL:	https://leetcode.com/problems/longest-common-prefix/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/longest-common-prefix.kt

@@ -0,0 +1,18 @@
+class Solution {
+    private fun findCommonPrefix(
+        x: String,
+        y: String,
+    ): String =
+        minOf(x.length, y.length).let { maxLength ->
+            (0..maxLength - 1).takeWhile { i ->
+                x[i] == y[i]
+            }.lastOrNull() ?: -1
+        }.let { maxIndex ->
+            x.substring(0, maxIndex + 1)
+        }
+
+    fun longestCommonPrefix(strs: Array<String>): String =
+        strs.drop(1).fold(strs[0]) { commonPrefix, str ->
+            findCommonPrefix(commonPrefix, str)
+        }
+}