diff --git a/Distinct Subsequences.py b/Distinct Subsequences.py
index 3ef3b6c..e7e2591 100644
--- a/Distinct Subsequences.py	
+++ b/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 original 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)]
\ No newline at end of file
+        return ways[len(T)][len(S)]
diff --git a/Excel Sheet Column Number.py b/Excel Sheet Column Number.py
new file mode 100644
index 0000000..c715f7c
--- /dev/null
+++ b/Excel Sheet Column Number.py	
@@ -0,0 +1,7 @@
+class Solution:
+    def titleToNumber(self, s):
+        num = 0
+        for i in s:
+            num *= 26
+            num += ord(i) - 64
+        return num
diff --git a/Largest Number.py b/Largest Number.py
new file mode 100644
index 0000000..e18d155
--- /dev/null
+++ b/Largest Number.py	
@@ -0,0 +1,5 @@
+class Solution:
+    def largestNumber(self, num):
+        num = [str(x) for x in num]
+        num.sort(cmp = lambda x, y: cmp(y+x, x+y))
+        return ''.join(num).lstrip('0') or '0'
diff --git a/Number of 1 Bits.py b/Number of 1 Bits.py
new file mode 100644
index 0000000..960e0bc
--- /dev/null
+++ b/Number of 1 Bits.py	
@@ -0,0 +1,7 @@
+class Solution:
+    def hammingWeight(self, n):
+        count = 0
+        while n:
+            count += 1
+            n = (n - 1) & n
+        return count
diff --git a/Remove Nth Node From End of List.py b/Remove Nth Node From End of List.py
index 582119d..4ca1817 100644
--- a/Remove Nth Node From End of List.py	
+++ b/Remove Nth Node From End of List.py	
@@ -1,11 +1,12 @@
 class Solution:
     def removeNthFromEnd(self, head, n):
-        fast, slow, prev = head, head, None
-        while n > 0:
-            fast, n = fast.next, n - 1
-        while fast != None:
-            fast, slow, prev = fast.next, slow.next, slow
-        if prev == None:
-            return head.next
-        prev.next = prev.next.next
-        return head
\ No newline at end of file
+        dummy = ListNode(-1)
+        dummy.next = head
+        left, right = dummy, dummy
+        while n:
+            right, n = right.next, n-1
+        while right.next: 
+            right = right.next
+            left = left.next
+        left.next = left.next.next
+        return dummy.next
diff --git a/Reverse Words in a String.py b/Reverse Words in a String.py
index cf23aff..0cd492d 100644
--- a/Reverse Words in a String.py	
+++ b/Reverse Words in a String.py	
@@ -1,3 +1,3 @@
 class Solution:
     def reverseWords(self, s):
-        return " ".join(filter(lambda x: x, reversed(s.split(" "))))
\ No newline at end of file
+        return " ".join(filter(None, reversed(s.split(" "))))
diff --git a/Word Break II.py b/Word Break II.py
index 97f94b4..680eccf 100644
--- a/Word Break II.py	
+++ b/Word Break II.py	
@@ -4,7 +4,6 @@ def wordBreak(self, s, dict):
         f = [False for _ in xrange(n)]
         trace = [[False] * n for _ in xrange(n)]
         for i in xrange(n):
-            trace.append([])
             if s[:i+1] in dict:
                 f[i] = True
                 trace[0][i] = True