From d3bc57de1822225a541a0981d0684d42cb3709f3 Mon Sep 17 00:00:00 2001
From: Matej Focko <me@mfocko.xyz>
Date: Mon, 25 Dec 2023 15:39:19 +0100
Subject: [PATCH] =?UTF-8?q?java:=20add=20=E2=80=9C91.=20Decode=20Ways?=
 =?UTF-8?q?=E2=80=9D?=
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Signed-off-by: Matej Focko <me@mfocko.xyz>
---
 java/decode-ways.java | 51 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)
 create mode 100644 java/decode-ways.java

diff --git a/java/decode-ways.java b/java/decode-ways.java
new file mode 100644
index 0000000..3bb9f71
--- /dev/null
+++ b/java/decode-ways.java
@@ -0,0 +1,51 @@
+class Solution {
+    private static final int EMPTY = -1;
+
+    private static int charToDigit(String s, int i) {
+        return s.charAt(i) - '0';
+    }
+
+    private int numDecodings(String s, int[] dp, int i) {
+        // out of bounds
+        if (i < 0 || i > s.length()) {
+            return 0;
+        }
+
+        // final digit
+        if (i == s.length()) {
+            return 1;
+        }
+
+        // starting with 0, not allowed
+        if (s.charAt(i) == '0') {
+            return 0;
+        }
+
+        // solution has been already precomputed
+        if (dp[i] != EMPTY) {
+            return dp[i];
+        }
+
+        // taking just one digit
+        int result = numDecodings(s, dp, i + 1);
+
+        // taking two digits
+        if (i + 1 < s.length() && charToDigit(s, i) * 10 + charToDigit(s, i + 1) <= 26) {
+            result += numDecodings(s, dp, i + 2);
+        }
+
+        // save and return
+        dp[i] = result;
+        return result;
+    }
+
+    public int numDecodings(String s) {
+        int[] dp = new int[s.length()];
+        for (int i = 0; i < dp.length; ++i) {
+            dp[i] = EMPTY;
+        }
+
+        return numDecodings(s, dp, 0);
+    }
+}
+