SwapNodesInPairs.java

/**
 * Given a linked list, swap every two adjacent nodes and return its head.
 * <p>
 * For example,
 * Given 1->2->3->4, you should return the list as 2->1->4->3.
 * <p>
 * Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.
 * <p>
 * Accepted.
 */
public class SwapNodesInPairs {

    public ListNode swapPairs(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode pre = head, nxt = pre.next;
        while (pre != null && nxt != null) {
            int tmp = nxt.val;
            nxt.val = pre.val;
            pre.val = tmp;

            pre = nxt.next;
            if (pre != null) {
                nxt = pre.next;
            }
        }

        return head;
    }

    public static class ListNode {

        int val;
        ListNode next;

        ListNode(int x) {
            val = x;
        }

        @Override
        public boolean equals(Object obj) {
            if (obj instanceof ListNode) {
                ListNode node = (ListNode) obj;
                return this.next == null && node.next == null || this.val == node.val && (this.next != null) && this.next.equals(node.next);
            }
            return false;
        }

    }

}