Reversed 2 linked list. Today we shared the song OldStreet. Time is r…

…eally slow on the old street.

206-reverse-linked-list-in-two-ways

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+# Definition for singly-linked list.
+# class ListNode(object):
+#     def __init__(self, x):
+#         self.val = x
+#         self.next = None
+
+# EASY
+# Iteratively: T:O(N) S:O(1)
+# Recursively: T:O(N) S:O(N)
+class Solution(object):
+    def reverseList(self, head):
+        """
+        :type head: ListNode
+        :rtype: ListNode
+        """
+        # Iteratively 
+        # rev = None
+        # while head!=None:
+        #     temp = head.next
+        #     head.next = rev
+        #     rev = head
+        #     head = temp
+        # return rev
+
+        # Recursively
+        return self._reverse(head,None)
+
+    def _reverse(self,head,prev):
+        if head == None:
+            return prev
+        else:
+            temp = head.next
+            head.next = prev
+            return self._reverse(temp,head)

92-reverse-linked-list-II

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+# Medium
+# O(N) O(1)
+
+
+# Definition for singly-linked list.
+# class ListNode(object):
+#     def __init__(self, x):
+#         self.val = x
+#         self.next = None
+
+class Solution(object):
+    def reverseBetween(self, head, m, n):
+        """
+        :type head: ListNode
+        :type m: int
+        :type n: int 
+        :rtype: ListNode
+        """
+        i = 0
+        if(m==n):
+            return head
+        dummy = ListNode(0)
+        dummy.next = head
+        ret = dummy
+        while dummy:
+            if i+1==m:
+                dummy.next = self.reverseBegin(dummy.next, n-i)
+                return ret.next
+            i += 1
+            dummy = dummy.next
+
+    def reverseBegin(self, node, k):
+        i = 0
+        rev = None
+        ret = node
+        while i<k:
+            i += 1
+            temp = node.next
+            node.next= rev
+            rev = node
+            node = temp
+        ret.next = temp
+        return rev
+