链表合并问题的合并排序的Java实现

Question

I've been looking at merge sort implementation in Java and don't understand the following part:

Node middle = getMiddle(head);      //get the middle of the list
Node left_head = head;
Node right_head = middle.next; 
middle.next = null;             //split the list into two half's

So by setting middle.next = null we split left_head by the middle. But why right_head remains in place since it refers to middle.next which became null ?

UPDATE: Ok, so what got me confused here is that middle.next = null doesn't actually sets next Node as a null, but updates middle's next variable to null , which of course doesn't affect right_head .

Answer 1

If you carefully observe the code segment you've posted, you'll see that the code executes line by line in this case. Following is a step by step description :

1) middle is set to point to the middle most node.
2) left head points to the first node in the left part of the split.
3) right head points to the first node in the right part of the split, which is actually the next to the middle node.
4) when middle.next is set to null, the chaining of nodes is broken and the linked list splits into two halves, where left_head and right_head , point to the corresponding left and right segments.

Answer 2

right_head was set before middle.next became null. So it holds the value of whatever was set before it becomes null.

You did not post the entire code segment, so I cannot confirm with certainty, but given merge sort 's desire to split the array into two pieces, sort them, and then combine them, it probably will reselect the middle element to which it should sort around for the next go, which is why it resets after this one.