java: add “91. Decode Ways”

Signed-off-by: Matej Focko <me@mfocko.xyz>

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);
+    }
+}
+