Solution for "Decode Ways"

Author: Thuy Trinh

src/main/java/leetcode/decodeways/README.md

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+https://leetcode.com/problems/decode-ways/description/
+
+A -> 1
+B -> 2
+C -> 3
+D -> 4
+E -> 5
+F -> 6
+...
+Z -> 26
+
+Some special cases:
+* 0 -> 0 way.
+* 01 -> 0 way.
+* 10 -> 1 way.
+* 20 -> 1 way.
+* 50 -> 0 way.
+* 150 -> 0 way.
+
+"12" -> AB | L -> 2 ways
+"126" -> ABF | AZ | LF -> 3 ways
+
+E.g. "12":
+Given f("12") which returns the total number ways to decode "12".
+* If we choose 1, we have one way to decode:
+  * 1 -> A
+  * 2 -> B
+* If we choose 12, we have one way to decode: 12 -> L.
+* Thus, the total number of ways to decode: 1 + 1 -> 2.
+
+E.g. "123":
+* If choosing 1 -> A, then we'll have to decode 23. There're 2 ways to decode 23. So the number of ways to decode is 2.
+* If choosing 12 -> L, then we'll decode 3 and we got only one way to do this. So the number of ways to decode is 1.
+* Thus, the total number of ways to decode is 2 + 1 -> 3.
+
+E.g. "1234":
+* 1,2,3,4
+* 12,3,4
+* 1,23,4
+-> 3 ways?
+
+d(s, i): Returns the number of ways to decode s.
+* i > s.lastIndex -> 0
+* Choose s[i] -> r += max(1, d(s, i + 1))
+* If s[i] and s[i + 1] can be decoded into an alphabet char, choose s[i] and s[i + 1] -> r += max(1, d(s, i + 2))

src/main/java/leetcode/decodeways/Solution.kt

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+package leetcode.decodeways
+
+/**
+ * https://leetcode.com/problems/decode-ways/description/
+ */
+class Solution {
+  fun numDecodings(s: String): Int = when {
+    s.isEmpty() -> 0
+    else -> maxOf(decodeWays(s), 0)
+  }
+
+  private fun decodeWays(
+      s: String,
+      i: Int = 0,
+      cache: MutableMap<Int, Int> = mutableMapOf()
+  ): Int = when {
+    i > s.lastIndex -> 0
+    s[i] == '0' -> -1
+    i == s.lastIndex -> 1
+    else -> cache.getOrPut(i) {
+      val w1 = decodeWays(s, i + 1, cache)
+      val w2 = if ("${s[i]}${s[i + 1]}".toInt() <= 26) {
+        decodeWays(s, i + 2, cache)
+      } else {
+        -1
+      }
+      when {
+        w1 == -1 && w2 == -1 -> -1
+        w1 == -1 -> maxOf(1, w2)
+        w2 == -1 -> maxOf(1, w1)
+        else -> maxOf(1, w1) + maxOf(1, w2)
+      }
+    }
+  }
+}

src/test/java/leetcode/decodeways/SolutionTest.kt

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+package leetcode.decodeways
+
+import org.amshove.kluent.shouldEqualTo
+import org.junit.Test
+
+class SolutionTest {
+  @Test
+  fun decodeWays() {
+    Solution().numDecodings("0").shouldEqualTo(0)
+    Solution().numDecodings("").shouldEqualTo(0)
+    Solution().numDecodings("2").shouldEqualTo(1)
+    Solution().numDecodings("12").shouldEqualTo(2)
+    Solution().numDecodings("123").shouldEqualTo(3)
+    Solution().numDecodings("1234").shouldEqualTo(3)
+    Solution().numDecodings("150").shouldEqualTo(0)
+    Solution().numDecodings("20").shouldEqualTo(1)
+    Solution().numDecodings("50").shouldEqualTo(0)
+  }
+}