Add an alternative solution

Author: yjc801

Distinct Subsequences.py

@@ -1,4 +1,17 @@
 class Solution:
+    def numDistinct(self, S, T):
+        n, m = len(S), len(T)
+        f = [0] * (m + 1)
+        f[0] = 1
+        for i in xrange(n):
+            for j in xrange(m - 1, -1, -1):
+                if S[i] == T[j]:
+                    f[j + 1] += f[j]
+        return f[m]
+
+class Solution:
+    ''' The alternative solution is to use a 2D array.
+    '''
     def numDistinct(self, S, T):
         ways = [[0 for j in range(len(S) + 1)] for i in range(len(T) + 1)]
         for i in range(len(S) + 1):
@@ -8,4 +21,4 @@ def numDistinct(self, S, T):
                 ways[i][j] = ways[i][j - 1]
                 if T[i - 1] == S[j - 1]:
                     ways[i][j] += ways[i - 1][j - 1]
-        return ways[len(T)][len(S)]
+        return ways[len(T)][len(S)]