LeetCode -- 24. Swap Nodes in Pairs

 

The meaning of problems and ideas

Meaning of the questions: two nodes exchange (not the value of the exchange, the exchange node), not twenty-two exchange, if there are four on the front of the exchange, the latter two (emmm, self-understanding, my expression is slightly pale) .

Thinking: Maintenance three indicator variables, respectively, first, p, q. Where the first core node, to avoid the exchange of data chaos. p, q are two nodes need to exchange. See in particular code.

Attach mind map

 

 

Code

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        if(head==null) return head; 
        ListNode first,p,q,top = new ListNode(-1);
        top.next = head;
        first = top; p = top.next; q = p.next;
        while(p!=null && q!=null){
            p.next = q.next;
            q.next = p;
            first.next = q;
            first = p;
            p = p.next;
            if(p==null) break;
            q = p.next;
        }
        return top.next;
    }
}