[hackerrank] Special String Again

Author: codehumane

src/main/kotlin/hackerrank/string/SpecialStringAgain.kt

@@ -0,0 +1,54 @@
+package hackerrank.string
+
+import kotlin.math.min
+
+fun substrCount(n: Int, s: String): Long {
+
+    var count = n.toLong()
+    val serial = arrayOfNulls<Char>(n)
+    var last = -1
+
+    s.forEachIndexed { idx, char ->
+        if (last >= 0 && serial[last] != char) {
+            count += countComposition(last + 1)
+            count += countMidException(idx, last, s, serial[last]!!)
+            last = -1
+        }
+
+        serial[++last] = char
+        if (idx == s.length - 1)
+            count += countComposition(last + 1)
+    }
+
+    return count
+}
+
+private fun countMidException(
+    idx: Int,
+    last: Int,
+    s: String, // 최초에 주어진 s
+    char: Char
+): Int {
+
+    var count = 0
+    val startIdx = idx + 1
+    if (startIdx >= s.length) return 0
+    val endIdx = min(idx + last + 1, s.length - 1)
+
+    (startIdx..endIdx).forEach {
+        if (s[it] == char) count++
+        else return count
+    }
+
+    return count
+}
+
+/**
+ * a -> 0
+ * aa -> 1
+ * aaa -> 2 + 1
+ * aaaa -> 3 + 2 + 1
+ * ...
+ * a가 n개 -> n(n - 1) / 2
+ */
+fun countComposition(elementCount: Int): Long = elementCount.toLong().let { it * (it - 1) / 2 }

src/test/kotlin/hackerrank/string/SpecialStringAgainKtTest.kt

@@ -0,0 +1,40 @@
+package hackerrank.string
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+class SpecialStringAgainKtTest {
+
+    @Test
+    fun `substrCount 모든 글자가 같은 경우를 고려해야 한다`() {
+        assertEquals(1, substrCount(1, "a"))
+        assertEquals(2, substrCount(2, "ab"))
+        assertEquals(2 + 1, substrCount(2, "aa"))
+        assertEquals(3 + 1, substrCount(3, "aab"))
+        assertEquals(3 + 3, substrCount(3, "aaa")) // (n - 1) + ... 1
+    }
+
+    @Test
+    fun `substrCount 가운데 글자를 제외한 모든 글자가 같은 경우도 고려해야 한다`() {
+        assertEquals(3 + 1, substrCount(3, "aba"))
+        assertEquals(4 + 1 + 1, substrCount(4, "abab"))
+        assertEquals(5 + 3 + 1, substrCount(5, "aaaba"))
+        assertEquals(6 + 3 + 1 + 2, substrCount(6, "aaabaa"))
+        assertEquals(7 + 3 + 3 + 3, substrCount(7, "aaabaaa"))
+    }
+
+    @Test
+    fun substrCount() {
+        assertEquals(1, substrCount(1, "a"))
+        assertEquals(2, substrCount(2, "ab"))
+        assertEquals(3, substrCount(2, "aa"))
+        assertEquals(4, substrCount(3, "aba"))
+        assertEquals(4, substrCount(3, "aab"))
+        assertEquals(4, substrCount(3, "abb"))
+        assertEquals(6, substrCount(3, "aaa"))
+        assertEquals(7, substrCount(5, "asasd"))
+        assertEquals(10, substrCount(7, "abcbaba"))
+        assertEquals(10, substrCount(4, "aaaa"))
+    }
+
+}