Create decodeWays

Author: kanglicheng

Kangli/DP/decodeWays

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+O(N) time, O(1) space DP via "general fibonacci numbers"
+class Solution(object):
+    def numDecodings(self, s):
+        if len(s) == 0 or s[0] == '0':
+            return 0
+        prev, prev_prev = 1, 0
+        for i in xrange(len(s)):
+            cur = 0
+            if s[i] != '0':
+                cur = prev
+            if i > 0 and (s[i - 1] == '1' or (s[i - 1] == '2' and s[i] <= '6')):
+                cur += prev_prev
+            prev, prev_prev = cur, prev
+        return prev
+                
+