How to reverse linked list in golang

Reversing a linked list is a common question and is often mentioned in programming interviews. It is a classic algorithmic problem that is widely used and can be used to quickly reverse the order of a linked list. This article will introduce the algorithm and steps to implement a reverse linked list using golang language.

1. Define a singly linked list node

Before we start to implement the reverse linked list, we need to first define a singly linked list node. A node contains two very important parts: data field and pointer field. The data field is used to store the value of the node, and the pointer field is used to point to the next node.

In golang, we can use the struct structure to define a singly linked list node. The structure contains two attributes: Val, used to represent the value of the current node, and Next, used to represent the pointer to the next node.

type ListNode struct {

Val  int
Next *ListNode

}

2. Singly linked list inversion

Now we have defined the nodes of the singly linked list, The next step is to implement the algorithm for inverting the linked list. The key to reversing a linked list is to traverse the linked list and change the pointer to each node.

We can traverse each node in the linked list from the beginning, and change their "Next" pointers in turn to point to the previous node. In this way, the linked list can be reversed.

The algorithm steps for reversing the linked list are as follows:

(1) Define two pointers: pre and cur, pointing to the first node and the second node respectively. pre is the previous node and cur is the current node.

(2) Traverse the linked list and point the Next pointer of the current node to the previous node pre.

(3) Move the pointer backward, point pre to the current node, and cur to the next node.

(4) Repeat steps 2 and 3 until the entire linked list is traversed.

The implementation code is as follows:

func reverseLinkedList(head ListNode) ListNode {

var pre *ListNode
cur := head
for cur != nil {
    next := cur.Next
    cur.Next = pre
    pre = cur
    cur = next
}
return pre

}

3. Reverse linked list Test code

In order to verify the correctness of the reversed linked list, we write some test code to execute.

func TestReverseLinkedList(t *testing.T) {

head := &ListNode{Val: 1}
node1 := &ListNode{Val: 2}
node2 := &ListNode{Val: 3}
node3 := &ListNode{Val: 4}
node4 := &ListNode{Val: 5}

head.Next = node1
node1.Next = node2
node2.Next = node3
node3.Next = node4

newHead := reverseLinkedList(head)

assert.Equal(t, newHead.Val, 5)
assert.Equal(t, newHead.Next.Val, 4)
assert.Equal(t, newHead.Next.Next.Val, 3)
assert.Equal(t, newHead.Next.Next.Next.Val, 2)
assert.Equal(t, newHead.Next.Next.Next.Next.Val, 1)

}

4. Reverse part of the linked list

In addition to reversing the entire In addition to linked lists, we can also reverse part of the linked list. For example, reverse the part from node m to node n in the linked list. We just need to make slight modifications based on reversing the entire linked list.

We can first traverse to the m-1th node, the pre pointer points to this node, and cur points to the m-th node. Then, we perform the steps of reversing the linked list until we reverse to the nth node.

The implementation code is as follows:

func reverseBetween(head ListNode, m int, n int) ListNode {

dummy := &ListNode{0, head}
pre := dummy

for i := 1; i < m; i++ {
    pre = pre.Next
}

cur := pre.Next
for i := m; i < n; i++ {
    next := cur.Next
    cur.Next = next.Next
    next.Next = pre.Next
    pre.Next = next
}

return dummy.Next

}

5. Test code for reversing partial linked list

In order to verify the correctness of reversing partial linked list, we write some test code for verification.

func TestReverseBetween(t *testing.T) {

head := &ListNode{Val: 1}
node1 := &ListNode{Val: 2}
node2 := &ListNode{Val: 3}
node3 := &ListNode{Val: 4}
node4 := &ListNode{Val: 5}

head.Next = node1
node1.Next = node2
node2.Next = node3
node3.Next = node4

newHead := reverseBetween(head, 2, 4)

assert.Equal(t, newHead.Val, 1)
assert.Equal(t, newHead.Next.Val, 4)
assert.Equal(t, newHead.Next.Next.Val, 3)
assert.Equal(t, newHead.Next.Next.Next.Val, 2)
assert.Equal(t, newHead.Next.Next.Next.Next.Val, 5)

}

6. Summary

In this article, we use golang Implemented the reversal linked list algorithm, including reversing the entire linked list and reversing part of the linked list. Reversing a linked list is a common interview question and is also a basic algorithm for solving problems related to linked lists. If you are interested in linked list algorithms, it is recommended that you study in depth other linked list related algorithms, such as fast and slow pointers, circular linked lists, deleting nodes, etc.

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