Trac #34343: Speed up computing outside corners of partition

Speed up the code for getting the corners of a partition.

URL: https://trac.sagemath.org/34343
Reported by: tkarn
Ticket author(s): Trevor K. Karn
Reviewer(s): Travis Scrimshaw

src/sage/combinat/partition.py

@@ -4027,15 +4027,12 @@ def outside_corners(self):
             sage: Partition([]).outside_corners()
             [(0, 0)]
         """
-        p = self
-        if p.is_empty():
+        p = self._list
+        if not p:
             return [(0,0)]
-        res = [ (0, p[0]) ]
-        for i in range(1, len(p)):
-            if p[i-1] != p[i]:
-                res.append((i,p[i]))
+        res = [(0, p[0])]
+        res.extend((n, j) for n, (i, j) in enumerate(zip(p[:-1], p[1:]), start=1) if i != j)
         res.append((len(p), 0))
-
         return res
 
     addable_cells = outside_corners   # for compatibility with partition tuples