diff --git a/README.md b/README.md
index 89f3b2d..80c88a5 100644
--- a/README.md
+++ b/README.md
@@ -74,8 +74,8 @@ Some pictures and ideas are from `<<Introduction to Algotithm>>`
         * [isBipartGraph.py](./graph/isBipartGraph.py)
     * [math](./math)
         * [README.md](./math/README.md)
+        * [dft.py](./math/dft.py)
         * [fastPow.py](./math/fastPow.py)
-        * [fft.py](./math/fft.py)
         * [fibonacci](./math/fibonacci)
         * [numWeight](./math/numWeight)
         * [numberTheory](./math/numberTheory)
diff --git a/math/fft.py b/math/dft.py
similarity index 100%
rename from math/fft.py
rename to math/dft.py
diff --git a/math/permute/permute_back_track.py b/math/permute/permute_back_track.py
index 84d6c15..89934e2 100644
--- a/math/permute/permute_back_track.py
+++ b/math/permute/permute_back_track.py
@@ -9,15 +9,19 @@
 # Description:
 #########################################################################
 '''
-def permute(n):
-    def _util(lst,i):
-        if i==n:print(lst)
-        else:
-            for j in range(i,n):
-                lst[i],lst[j]=lst[j],lst[i]
-                _util(lst,i+1)
-                lst[i],lst[j]=lst[j],lst[i]
-    _util([i for i in range(n)],0)
 
-if __name__=='__main__':
+
+def permute(n):
+    def _util(lst, i):
+        if i == n:
+            print(lst)
+        else:
+            for j in range(i, n):
+                lst[i], lst[j] = lst[j], lst[i]
+                _util(lst, i+1)
+                lst[i], lst[j] = lst[j], lst[i]
+    _util([i for i in range(n)], 0)
+
+
+if __name__ == '__main__':
     permute(5)
diff --git a/math/permute/permute_divide_and_conquer.py b/math/permute/permute_divide_and_conquer.py
index 922f04c..e90f624 100644
--- a/math/permute/permute_divide_and_conquer.py
+++ b/math/permute/permute_divide_and_conquer.py
@@ -9,15 +9,19 @@
 # Description:
 #########################################################################
 '''
-def permute(lst,n):
+
+
+def permute(lst, n):
     ''' O(n!), optimal'''
-    if n==1:print(lst)
+    if n == 1:
+        print(lst)
     else:
         for i in range(n):
-            lst[i],lst[n-1] = lst[n-1],lst[i]
-            permute(lst,n-1)
-            lst[i],lst[n-1] = lst[n-1],lst[i]
+            lst[i], lst[n-1] = lst[n-1], lst[i]
+            permute(lst, n-1)
+            lst[i], lst[n-1] = lst[n-1], lst[i]
 
-if __name__=='__main__':
+
+if __name__ == '__main__':
     n = 3
-    permute([i for i in range(n)],n)
+    permute([i for i in range(n)], n)
diff --git a/poly.c b/poly.c
new file mode 100644
index 0000000..e69de29