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Leetcode[143] Reorder List

11 Nov 2015

###Task1 Given a singly linked list L: L0→L1→…→Ln-1→Ln, reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…

You must do this in-place without altering the nodes’ values.

For example, Given {1,2,3,4}, reorder it to {1,4,2,3}. ###Python

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def reorderList(self, head):
        """
        :type head: ListNode
        :rtype: void Do not return anything, modify head in-place instead.
        """
        if not head:
            return
        mid = self.findMid(head)
        second = mid.next
        mid.next = None
        secondR = self.reverse(second)
        newHead = self.merge(head, secondR)
        return newHead
    
    def merge(self, first, second):
        dummy = ListNode(0)
        turn = True
        while first and second:
            if turn:
                dummy.next = first
                dummy = dummy.next
                first = first.next
                turn = not turn
            else:
                dummy.next = second
                dummy = dummy.next
                second = second.next
                turn = not turn
        if first:
            dummy.next = first
            dummy = dummy.next
            first = first.next
        if second:
            dummy.next = second
            dummy = dummy.next
            second = second.next
        return dummy.next
        
    def reverse(self, head):
        prev = None
        while head:
            temp = head.next
            head.next = prev
            prev = head
            head = temp
        return prev
    
    def findMid(self, head):
        slow = head
        fast = head.next
        while fast and fast.next:
            prev = slow
            slow = slow.next
            fast = fast.next.next
        return slow

###Java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    
    public ListNode findMiddle(ListNode head) {
        ListNode slow = head;
        ListNode fast = head.next;
        
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }
        
        return slow;
    }
    
    public ListNode reverse(ListNode head) {
        ListNode prev = null;
        
        while (head != null) {
            ListNode temp = head.next;
            head.next = prev;
            prev = head;
            head = temp;
        }
        
        return prev;
    }
    
    
    public void reorderList(ListNode head) {
        if (head == null || head.next == null) {
            return;
        }
        
        ListNode dummy = new ListNode(0);
        ListNode parser = dummy;
        
        ListNode mid = findMiddle(head);
        ListNode reversed = reverse(mid.next);
        mid.next = null;
        
        boolean choice = true;
        while (head != null && reversed != null) {
            if (choice) {
                parser.next = head;
                head = head.next;
                choice = !choice;
            } else {
                parser.next = reversed;
                reversed = reversed.next;
                choice = !choice;
            }
            parser = parser.next;
        }
        
        if (head != null) {
            parser.next = head;
        }
        if (reversed != null) {
            parser.next = reversed;
        }
        
        return ;
    }
}

###Points

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