Merge K Sorted Lists

将多个有序链表拼接成一个，思路同拼接两个链表，先比较两个（或k个）链表的表头，最小的表头节点摘下，继续比较，直到所有链表的节点都被摘空为止。基于算法的效率考虑，每次比较k个链表的表头，只有最小的表头节点被拿走，下一次拿该表头节点的下一个节点和其余k-1个比较。此时对k个节点的排序不用堆排序都对不起当年在算法课上猛打瞌睡的自己。
代码如下：

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; } * } */public class Solution {    ArrayList<ListNode> nodeListHeap=new ArrayList<ListNode>();    public ListNode mergeKLists(ListNode[] lists) {        ListNode node=new ListNode(0);        ListNode head=node;        if(lists==null){            return null;        }        createHeap(lists);        while(nodeListHeap.size()>1){            if(nodeListHeap.get(0)==null){                return null;            }            node.next=nodeListHeap.get(0);            node=node.next;            //此处处理的要点是截断不能破坏堆结构，使用策略用常见的堆策略，即把最后一个放到第一个位置            if(nodeListHeap.get(0).next==null){                nodeListHeap.set(0,nodeListHeap.get(nodeListHeap.size()-1));                nodeListHeap.remove(nodeListHeap.size()-1);            }else{                nodeListHeap.set(0,nodeListHeap.get(0).next);            }            adjustHeap(0);        }        if(nodeListHeap.size()==1){            node.next=nodeListHeap.get(0);        }        return head.next;    }    public void createHeap(ListNode[] lists){            for(ListNode node:lists){            if(node!=null){                nodeListHeap.add(node);            }        }        if(nodeListHeap.size()<=1){            return;        }        int index=(nodeListHeap.size()-1)/2;        while(index>=0){            int leftparent=index*2+1;            int rightparent=index*2+2;            ListNode tempNode=null;            if(rightparent<nodeListHeap.size()&&nodeListHeap.get(leftparent).val>nodeListHeap.get(rightparent).val){                if(nodeListHeap.get(index).val>nodeListHeap.get(rightparent).val){                    ListNode nodetemp=nodeListHeap.get(index);                    nodeListHeap.set(index, nodeListHeap.get(rightparent));                    nodeListHeap.set(rightparent, nodetemp);                    adjustHeap(rightparent);                }            }else if(leftparent<nodeListHeap.size()&&nodeListHeap.get(index).val>nodeListHeap.get(leftparent).val){                ListNode nodetemp=nodeListHeap.get(index);                nodeListHeap.set(index, nodeListHeap.get(leftparent));                nodeListHeap.set(leftparent, nodetemp);                adjustHeap(leftparent);             }            index--;        }    }    public void adjustHeap(int start){        int index=start;        while(index<(nodeListHeap.size()+1)/2){            int leftparent=index*2+1;            int rightparent=index*2+2;            if(rightparent<nodeListHeap.size()&&nodeListHeap.get(leftparent).val>nodeListHeap.get(rightparent).val){                if(nodeListHeap.get(index).val>nodeListHeap.get(rightparent).val){                    ListNode nodetemp=nodeListHeap.get(index);                    nodeListHeap.set(index, nodeListHeap.get(rightparent));                    nodeListHeap.set(rightparent, nodetemp);                    index=rightparent;                }else{                    return;                }            }else if(leftparent<nodeListHeap.size()){                if(nodeListHeap.get(index).val>nodeListHeap.get(leftparent).val){                    ListNode nodetemp=nodeListHeap.get(index);                    nodeListHeap.set(index, nodeListHeap.get(leftparent));                    nodeListHeap.set(leftparent, nodetemp);                    index=leftparent;                }else{                    return;                }            }else{                return;            }        }        return;    }}