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ReorderList.java
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ReorderList.java
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package com.dbc;
public class ReorderList {
public class ListNode {
int val;
ListNode next;
ListNode() {
}
ListNode(int val) {
this.val = val;
}
ListNode(int val, ListNode next) {
this.val = val;
this.next = next;
}
}
public void reorderList(ListNode head) {
if (head == null) {
return;
}
ListNode mid = middleNode(head);
ListNode l1 = head;
ListNode l2 = mid.next;
mid.next = null;
l2 = reverseList(l2);
mergeList(l1, l2);
}
public ListNode middleNode(ListNode head) {
ListNode slow = head;
ListNode fast = head;
while (fast.next != null && fast.next.next != null) {
slow = slow.next;
fast = fast.next.next;
}
return slow;
}
public ListNode reverseList(ListNode head) {
ListNode prev = null;
ListNode curr = head;
while (curr != null) {
ListNode nextTemp = curr.next;
curr.next = prev;
prev = curr;
curr = nextTemp;
}
return prev;
}
public void mergeList(ListNode l1, ListNode l2) {
ListNode l1_tmp;
ListNode l2_tmp;
while (l1 != null && l2 != null) {
l1_tmp = l1.next;
l2_tmp = l2.next;
l1.next = l2;
l1 = l1_tmp;
l2.next = l1;
l2 = l2_tmp;
}
}
}