Completed Coursera: Kotlin for developers course

src/In.kt

@@ -0,0 +1,24 @@
+import java.time.Instant
+
+fun main() {
+    // Check if a is in abc
+    println('a' in "abc")
+    // Check if a is not in abc
+    println('a' !in "abc")
+    // Check if now is within the bounds, using comparable under the hood
+    println(Instant.now() in Instant.MIN..Instant.MAX)
+}
+
+/// Playground
+fun isValidIdentifier(s: String): Boolean {
+    fun isValidCharacter(ch: Char): Boolean = ch == '_' || ch.isLetterOrDigit()
+    if (s.isEmpty() || s[0].isDigit()) {
+        return false
+    }
+    for (ch in s) {
+        if (!isValidCharacter(ch)) {
+            return false
+        }
+    }
+    return true
+}