Interview question: Merge two sorted linked list without creating a new list [closed]

问题

How to merge sorted list l1 and l2 recursively?

回答1:

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    if (l1 == null) {
        return l2;
    }
    if (l2 == null) {
        return l1;
    }
    ListNode result = l1.val > l2.val ? l2 : l1;
    ListNode cur = result;
    ListNode h1 = result.next;
    ListNode h2 = result == l1 ? l2 : l1;
    while (h1 != null && h2 != null) {
        if (h2.val <= h1.val) {
            cur.next = h2;
            cur = cur.next;
            h2 = h2.next;
        } else {
            cur.next = h1;
            cur = cur.next;
            h1 = h1.next;
        }
    }
    while (h1 != null) {
        cur.next = h1;
        cur = cur.next;
        h1 = h1.next;
    }
    while (h2 != null) {
        cur.next = h2;
        cur = cur.next;
        h2 = h2.next;
    }
    return result;
}

回答2:

You need a language with call by reference parameters for this problem to make any sense. Even so, you wouldn't want to solve a problem like this recursively. I'd look for another employer.

In C++:

struct Node {
  int key;
  struct Node *next;
};

void merge(struct Node*& l1, struct Node*& l2) {
  // If at the end of l2, we're done.
  if (!l2) return;

  // If at the end of l1, move all of l2 to l1, then we're done. 
  if (!l1) {
    l1 = l2;
    l2 = 0;
    return;
  }
  // If the head of l2 is smaller than the head of l1, pop from l2 and push on l1.
  if (l2->key < l1->key) {
    // Pop.
    Node *l2saved = l2;
    l2 = l2saved->next;
    // Push.
    l2saved->next = l1;
    l1 = l2saved;
   }
   // Recur skipping the node we just merged.
   merge(l1->next, l2);
}

For C, just use pointers to pointers instead of references to pointers. Not surprisingly, with optimization turned up clang compiles this to a tight loop.

来源：https://stackoverflow.com/questions/59780599/interview-question-merge-two-sorted-linked-list-without-creating-a-new-list

标签

algorithm

data-structures

merge

mergesort