decode ways solution

Author: Jeongwon Na

decode-ways/njngwn.java

@@ -0,0 +1,33 @@
+// Time Complexity: O(n), n: s.length()
+// Space Complexity: O(n), n: s.length()
+class Solution {
+    public int numDecodings(String s) {
+        if (s.length() == 0) {  // edge case
+            return 0;
+        }
+
+        int length = s.length()+1;
+        int[] cntArr = new int[length]; // using dynamic programming
+
+        // check the case i == 0, i == 1 first
+        cntArr[0] = 1;
+        if (s.charAt(0) != '0') {
+            cntArr[1] = 1;
+        }
+
+        for (int i = 2; i < length; ++i) {
+            char ch = s.charAt(i-1);
+            if (ch != '0') {   // check for 1-9
+                cntArr[i] += cntArr[i-1];
+            }
+
+            // check for 10-26
+            int num = (s.charAt(i-2)-'0') * 10 + (ch-'0');
+            if (num >= 10 && num <= 26) {
+                cntArr[i] += cntArr[i-2];
+            }
+        }
+
+        return cntArr[length-1];
+    }
+}