diff --git a/Lib/posixpath.py b/Lib/posixpath.py
index 7f907ef..7ee4911 100644
--- a/Lib/posixpath.py
+++ b/Lib/posixpath.py
@@ -123,16 +123,13 @@
 def commonprefix(m):
     "Given a list of pathnames, returns the longest common leading component"
     if not m: return ''
-    prefix = m[0]
-    for item in m:
-        for i in range(len(prefix)):
-            if prefix[:i+1] != item[:i+1]:
-                prefix = prefix[:i]
-                if i == 0:
-                    return ''
-                break
-    return prefix
-
+    s1 = min(m)
+    s2 = max(m)
+    n = min(len(s1), len(s2))
+    for i in xrange(n):
+        if s1[i] != s2[i]:
+            return s1[:i]
+    return s1[:n]
 
 # Get size, mtime, atime of files.