Coursera Algorithms week3 归并排序 练习测验: Shuffling a linked list

题目原文：

Shuffling a linked list. Given a singly-linked list containing n items, rearrange the items uniformly at random. Your algorithm should consume a logarithmic (or constant) amount of extra memory and run in time proportional to nlogn in the worst case.

分析：

此题要求对单向链表进行随机排序，可以考虑先实现个链表的归并排序(见我的另一篇文章)，再改造成这里的随机排序。

 import java.util.Iterator;
 import edu.princeton.cs.algs4.StdRandom;
 /**
  /**
  * 对单向链表的随机归并排序
  * @author evasean www.cnblogs.com/evasean/
  * @param <T>
  */
 public class ShuffleLinkedList<T extends Comparable<T>> implements Iterable<T>{
     private Node first = null;
     private Node last = null;
     private int n;
     private class Node{
         T element;
         Node next;
     }
     private boolean less(Comparable v, Comparable w) {
         return v.compareTo(w) < 0;
     }
     @Override
     public Iterator<T> iterator() {
         // TODO Auto-generated method stub
         return new ListIterator();
     }
     private class ListIterator implements Iterator<T>{
         private Node current = first;
         @Override
         public boolean hasNext() {
             // TODO Auto-generated method stub
             return current != null;
         }

         @Override
         public T next() {
             // TODO Auto-generated method stub
             T t = current.element;
             current = current.next;
             return t;
         }
     }
     public void add(T t){
         Node node = new Node();
         node.element = t;
         node.next = null;
         if(first == null && last == null){
             first = node;
             last = node;
         }else if(first != null && first == last){
             first.next = node;
             last = node;
         }else{
             last.next = node;
             last = node;
         }
         n++;
     }
     @Override
     public String toString(){
         Iterator<T> iter = iterator();
         String ret = iter.next().toString();
         while(iter.hasNext()){
             ret += ", "+ iter.next().toString() ;
         }
         return ret;
     }
     public void mergeSort(){
         first = sort(first);
     }

     private Node sort(Node head){
         if(head == null || head.next == null) return head;
         Node slow = head;
         Node fast = head;
         //取中间节点
         while(fast.next != null && fast.next.next != null){
             slow = slow.next;
             fast = fast.next.next;
         }
         Node left = head;
         Node right = slow.next;
         slow.next = null; //将左右链表分开
         left = sort(left);
         right = sort(right);
         return merge(left,right);
     }
     private Node merge(Node left, Node right){
         //System.out.println("left="+left.element+",right="+right.element);
         Node aux = new Node(); //需要耗费logn的额外空间
         Node l= left;
         Node r = right;
         Node current = aux;
         while(l != null && r!=null){
             int rand = StdRandom.uniform(2);
             if(rand == 0){ //如果随机数选为0，则从左侧选取元素
                 current.next = l;
                 current = current.next;
                 l= l.next;
             }else{ //如果随机数选为1，则从右侧选取元素
                 current.next = r;
                 current = current.next;
                 r = r.next;
             }
         }
         if(l!=null) current.next = l; // 如果左侧没遍历完，将其连接至current后
         else if(r != null) current.next = r; //如果右侧没遍历完，将其连接至current后
         return aux.next; //返回归并好的链表
     }
      public static void main(String[] args){
          ShuffleLinkedList<Integer> sll = new ShuffleLinkedList<Integer>();
          sll.add(1);
          sll.add(2);
          sll.add(11);
          sll.add(9);
          sll.add(10);
          sll.add(4);
          sll.add(7);
          System.out.println(sll);
          sll.mergeSort();
          System.out.println(sll);
      }

 }