Solution for Decode Ways #268

Author: dohyeon2

decode-ways/dohyeon2.java

@@ -0,0 +1,35 @@
+class Solution {
+    // TC : O(n)
+    // SC : O(1)
+    public int numDecodings(String s) {
+        // If it starts with 0, the value can't be decoded.
+        if (s.charAt(0) == '0') {
+            return 0;
+        }
+
+        int n = s.length();
+
+        int prev2 = 1;
+        int prev1 = 1;
+
+        // dp[i] = dp[i-1] + dp[i-2]
+        for (int i = 1; i < n; i++) {
+            int current = 0;
+
+            if (s.charAt(i) != '0') {
+                current += prev1;
+            }
+
+            int twoDigit = Integer.valueOf(s.substring(i - 1, i + 1));
+
+            if (twoDigit >= 10 && twoDigit <= 26) {
+                current += prev2;
+            }
+
+            prev2 = prev1;
+            prev1 = current;
+        }
+
+        return prev1;
+    }
+}