合并两个或多个有序链表

合并两个有序链表：

法一：迭代

package main

func mergeTwoLists(list1 *ListNode, list2 *ListNode) *ListNode {
   dummyHead := &ListNode{}
   cursor := dummyHead
   for list1 != nil && list2 != nil {
      if list1.Val < list2.Val {
         cursor.Next = list1
         list1 = list1.Next
      } else {
         cursor.Next = list2
         list2 = list2.Next
      }
      cursor = cursor.Next
   }
   if list1 == nil {
      cursor.Next = list2
   } else {
      cursor.Next = list1
   }
   return dummyHead.Next
}　

法二：递归

package main

func mergeTwoLists(list1 *ListNode, list2 *ListNode) *ListNode {
   if list1 == nil {
      return list2
   } else if list2 == nil {
      return list1
   } else if list1.Val < list2.Val {
      list1.Next = mergeTwoLists(list1.Next, list2)
      return list1
   } else {
      list2.Next = mergeTwoLists(list1, list2.Next)
      return list2
   }
}

　　

合并多个有序链表：

法一：分治法

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func mergeKLists(lists []*ListNode) *ListNode {
    var pre, cur *ListNode
    n := len(lists)
    for i := 0; i < n; i++ {
        if i == 0 {
            pre = lists[i]
            continue
        }
        cur = lists[i]
        pre = merge(pre, cur)
    }
    return pre
}

func merge(l1, l2 *ListNode) *ListNode {
    head := &ListNode{}
    cur := head
    for l1 != nil || l2 != nil {
        if l1 != nil && l2 != nil {
            if l1.Val < l2.Val {
                cur.Next = l1
                l1 = l1.Next
            } else {
                cur.Next = l2
                l2 = l2.Next
            }
            cur = cur.Next
        } else if l1 != nil {
            cur.Next = l1
            break
        } else {
            cur.Next = l2
            break
        }
    }
    return head.Next
}

　法二：小根堆

func mergeKLists(lists []*ListNode) *ListNode {
	k := len(lists)
	h := new(minHeap)
	for i := 0; i < k; i++ {
		if lists[i] != nil {
			heap.Push(h, lists[i])
		}
	}

	dummyHead := new(ListNode)
	pre := dummyHead
	for h.Len() > 0 {
		tmp := heap.Pop(h).(*ListNode)
		if tmp.Next != nil {
			heap.Push(h, tmp.Next)
		}

		pre.Next = tmp
		pre = pre.Next
	}

	return dummyHead.Next
}

type minHeap []*ListNode

func (h minHeap) Len() int           { return len(h) }
func (h minHeap) Less(i, j int) bool { return h[i].Val < h[j].Val }
func (h minHeap) Swap(i, j int)      { h[i], h[j] = h[j], h[i] }
func (h *minHeap) Push(x interface{}) {
	// Push and Pop use pointer receivers because they modify the slice's length,
	// not just its contents.
	*h = append(*h, x.(*ListNode))
}
func (h *minHeap) Pop() interface{} {
	old := *h
	n := len(old)
	x := old[n-1]
	*h = old[0 : n-1]
	return x
}