Update decode_ways.cpp

Author: caba5

cpp/neetcode_150/13_1-d_dynamic_programming/decode_ways.cpp

@@ -8,7 +8,7 @@
     Time: O(n)
     Space: O(n)
 */
-
+/*
 class Solution {
 public:
     int numDecodings(string s) {
@@ -36,3 +36,41 @@ class Solution {
         return dp[n];
     }
 };
+*/
+
+/*
+    Given a string w/ only digits, return # ways to decode it (letter -> digit)
+    Ex. s = "12" -> 2 (AB 1 2 or L 12), s = "226" -> 3 (2 26 or 22 6 or 2 2 6)
+
+    DP: Bottom-up approach: check if zero and assign the i+1 value to the current;
+    otherwise, check if the current char is a '1' and thus admits any i+1 char,
+    or a '2', which admits only 0 <= x <= 6 as characters at i+1.
+
+    Time: O(n)
+    Space: O(1)
+*/
+class Solution {
+public:
+    int numDecodings(string s) {   
+        int n = s.size();     
+        int prev = 0;
+        int curr = 1;
+
+        for (int i = n - 1; i >= 0; --i){
+            int newCurr;
+            if (s[i] == '0')
+                newCurr = 0;
+            else
+                newCurr = curr;
+            
+            if (i + 1 < n && (s[i] == '1' || s[i] == '2' 
+                && s[i+1] <= '6'))
+                newCurr += prev;
+            
+            prev = curr;
+            curr = newCurr;
+        }
+
+        return curr;
+    }
+};