Given a linked list, swap every two adjacent nodes and return its head.
1->2->3->4, you should return the list as
Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.
NOTE: You only need to implement the given function. Do not read input, instead use the arguments to the function. Do not print the output, instead return values as specified. Still have a question? Checkout Sample Codes for more details.