O(n) time and O(1) space

Author: 149ps

160. Intersection of Two Linked Lists/160. Intersection of Two Linked Lists.py

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+"""
+Write a program to find the node at which the intersection of two singly linked lists begins.
+
+For example, the following two linked lists:
+
+
+begin to intersect at node c1.
+
+ 
+
+Example 1:
+
+Input: intersectVal = 8, listA = [4,1,8,4,5], listB = [5,0,1,8,4,5], skipA = 2, skipB = 3
+Output: Reference of the node with value = 8
+Input Explanation: The intersected node's value is 8 (note that this must not be 0 if the two lists intersect). From the head of A, it reads as [4,1,8,4,5]. From the head of B, it reads as [5,0,1,8,4,5]. There are 2 nodes before the intersected node in A; There are 3 nodes before the intersected node in B.
+"""
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.next = None
+
+class Solution:
+    def getIntersectionNode(self, headA: ListNode, headB: ListNode) -> ListNode:
+        a,b = headA,headB
+        lenA,lenB = 0,0
+        while a:
+            lenA += 1
+            a = a.next
+        while b:
+            lenB += 1
+            b = b.next
+        a,b = headA,headB
+        if lenA > lenB:
+            for i in range(lenA-lenB):
+                a = a.next
+        elif lenB > lenA:
+            for i in range(lenB-lenA):
+                b = b.next
+        while a != b:
+            a = a.next
+            b = b.next
+        return b