LeetCode – Swap Nodes in Pairs (Java)

Given a linked list, swap every two adjacent nodes and return its head.

For example, given 1->2->3->4, you should return the list as 2->1->4->3.

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

Java Solution 1

Use two template variable to track the previous and next node of each pair.

public ListNode swapPairs(ListNode head) {
    if(head == null || head.next == null)   
        return head;
 
    ListNode h = new ListNode(0);
    h.next = head;
    ListNode p = h;
 
    while(p.next != null && p.next.next != null){
        //use t1 to track first node
        ListNode t1 = p;
        p = p.next;
        t1.next = p.next;
 
        //use t2 to track next node of the pair
        ListNode t2 = p.next.next;
        p.next.next = p;
        p.next = t2;
    }
 
    return h.next;
}

Java Solution 2

Each time I do the same problem I often get the different solutions. Here is another way of writing this solution.

public ListNode swapPairs(ListNode head) {
    if(head==null || head.next==null)
        return head;
 
    //a fake head
    ListNode h = new ListNode(0);
    h.next = head;
 
    ListNode p1 = head;
    ListNode p2 = head.next;
 
    ListNode pre = h;
    while(p1!=null && p2!=null){
        pre.next = p2;
 
        ListNode t = p2.next;
        p2.next = p1;
        pre = p1;
        p1.next = t;
 
        p1 = p1.next;
 
        if(t!=null)
            p2 = t.next;
    }
 
    return h.next;
}

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Alik Elzin

–
Might be more a more clearer solution:
Node swapPairs(Node head) { Node headHelper = new Node(); headHelper.next = head;
/* prev node before next pair */ Node prev = headTemp; /* Has next pair */ while (prev.next != null && prev.next.next != null) { /* Start of pair */ Node curr = prev.next; /* Remember the next Pair */ Node nextPair = curr.next.next; /* Swap */ prev.next = curr.next; prev.next.next = curr; curr.next = nextPair; /* Iterate to next */ prev = curr; }
return headHelper.next; }
Mayank Mittal

100% on leet code:
class Solution { public ListNode swapPairs(ListNode head) {
ListNode helper = new ListNode(0); helper.next = head; ListNode previous = helper; ListNode current = head; while(current != null){ ListNode next = current.next; if(next!=null){ current.next = next.next; previous.next = next; next.next = current; previous = previous.next.next; current = current.next; }else{ current = current.next; } }
return helper.next; } }
Pop Adrian

No it doesn’t. Just tested both solution by hand and also on leetcode.
They are both correct.
Tài Nguyễn Thành

in while loop : p.next != null is the same head == null in if condition ahead
Harsh

Wow! Beautiful.
Chungho Song

public static ListNode swapPairs(ListNode head) { if(head == null || head.next == null) return head; else{ // swap the two ListNode newHead = head.next; head.next = swapPairs(head.next.next); newHead.next = head; return newHead; } }
Abhishek Jaiswal

class Solution {
public:
ListNode* swapPairs(ListNode* head) {

if(!head || !head->next) return head;

ListNode* send = head->next->next;
ListNode* temp = head->next;

temp->next = head;

head->next = swapPairs(send);

return temp;
}
};
Aaron Ke

hey dude, p.next.next = p; should change to t1.next.next=p;
ishan73

if(head==null || head.next==null){

return head;

}

ListNode slow=head.next;

head.next=head.next.next;

slow.next=head;

head=slow;

ListNode parent=slow.next;

slow=slow.next.next;

while(slow!=null && slow.next!=null){

ListNode temp=slow.next;

slow.next=slow.next.next;

temp.next=slow;

parent.next=temp;

parent=parent.next.next;

slow=slow.next;

}

return head;
Anonymous

public ListNode swapPairs(ListNode head) {
if(head == null || head.next == null){
return head;
}

ListNode next = swapPairs(head.next.next);
ListNode temp = head.next;
head.next = next;
temp.next = head;
return temp;
}
Sushant Raj

For Input–1211—>12—>13—>124—>121—>125—>
output—>1211—>124—>13—>125—>121—>
Sushant Raj

This code is not working…It removes the alternate element from the list..

LeetCode – Swap Nodes in Pairs (Java)

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