原题链接: https://oj.leetcode.com/problems/merge-two-sorted-lists/

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    if (l1 == null) return l2;
    if (l2 == null) return l1;

    ListNode cursorLeft = l1;
    ListNode cursorRight = l2;
    ListNode prevLeft = null;

    while (cursorRight != null) {
        while (cursorLeft != null && cursorLeft.val <= cursorRight.val) {
            prevLeft = cursorLeft;
            cursorLeft = cursorLeft.next;
        }

        if (prevLeft == null) {
            l2 = cursorRight.next;
            cursorRight.next = l1;
            l1 = cursorRight;
            cursorRight = l2;
        } else if (cursorLeft == null) {
            l2 = cursorRight.next;
            cursorRight.next = null;
            prevLeft.next = cursorRight;
            cursorRight = l2;
        } else {
            l2 = cursorRight.next;
            cursorRight.next = cursorLeft;
            prevLeft.next = cursorRight;
            cursorRight = l2;
        }

        cursorLeft = l1;
    }

    return l1;
}