Create iterator-for-combination.py

Author: kamyu104

Python/iterator-for-combination.py

@@ -0,0 +1,93 @@
+# Time:  O(k), per operation
+# Space: O(k)
+
+import itertools
+
+
+class CombinationIterator(object):
+
+    def __init__(self, characters, combinationLength):
+        """
+        :type characters: str
+        :type combinationLength: int
+        """
+        self.__it = itertools.combinations(characters, combinationLength)
+        self.__curr = None
+        self.__last = characters[-combinationLength:]
+
+    def next(self):
+        """
+        :rtype: str
+        """
+        self.__curr = "".join(self.__it.next())
+        return self.__curr
+    
+    def hasNext(self):
+        """
+        :rtype: bool
+        """
+        return self.__curr != self.__last
+
+
+# Time:  O(k), per operation
+# Space: O(k)
+import functools
+
+
+class CombinationIterator2(object):
+
+    def __init__(self, characters, combinationLength):
+        """
+        :type characters: str
+        :type combinationLength: int
+        """
+        self.__characters = characters
+        self.__combinationLength = combinationLength
+        self.__it = self.__iterative_dfs()
+        self.__curr = None
+        self.__last = characters[-combinationLength:]
+        
+    def __iterative_dfs(self):
+        def conquer():
+            if len(curr) == self.__combinationLength:
+                return curr
+
+        def prev_divide(c):
+            curr.append(c)
+        
+        def divide(i):
+            if len(curr) != self.__combinationLength:
+                for j in reversed(xrange(i, len(self.__characters)-(self.__combinationLength-len(curr)-1))):
+                    stk.append(functools.partial(post_divide))
+                    stk.append(functools.partial(divide, j+1))
+                    stk.append(functools.partial(prev_divide, self.__characters[j]))
+            stk.append(functools.partial(conquer))
+
+        def post_divide():
+            curr.pop()
+            
+        curr = []
+        stk = [functools.partial(divide, 0)]
+        while stk:
+            result = stk.pop()()
+            if result is not None:
+                yield result
+
+    def next(self):
+        """
+        :rtype: str
+        """
+        self.__curr = "".join(next(self.__it))
+        return self.__curr
+        
+    def hasNext(self):
+        """
+        :rtype: bool
+        """
+        return self.__curr != self.__last
+
+
+# Your CombinationIterator object will be instantiated and called as such:
+# obj = CombinationIterator(characters, combinationLength)
+# param_1 = obj.next()
+# param_2 = obj.hasNext()