Update coin_change.py (#706)

Cleanup PEP-8 , additional comments , using coin_val, instead of range and index.

dynamic_programming/coin_change.py

@@ -6,21 +6,26 @@ the given types of coins?
 https://www.hackerrank.com/challenges/coin-change/problem
 """
 from __future__ import print_function
+
+
 def dp_count(S, m, n):
+
+    # table[i] represents the number of ways to get to amount i
     table = [0] * (n + 1)
 
-    # Base case (If given value is 0)
+    # There is exactly 1 way to get to zero(You pick no coins).
     table[0] = 1
 
     # Pick all coins one by one and update table[] values
     # after the index greater than or equal to the value of the
     # picked coin
-    for i in range(0, m):
-        for j in range(S[i], n + 1):
-            table[j] += table[j - S[i]]
+    for coin_val in S:
+        for j in range(coin_val, n + 1):
+            table[j] += table[j - coin_val]
 
     return table[n]
 
+
 if __name__ == '__main__':
     print(dp_count([1, 2, 3], 3, 4))  # answer 4
     print(dp_count([2, 5, 3, 6], 4, 10))  # answer 5