Sort List LeetCode Solution

Problem – Sort List LeetCode Solution

Given the head of a linked list, return the list after sorting it in ascending order.

Example 1:

Input: head = [4,2,1,3]
Output: [1,2,3,4]

Example 2:

Input: head = [-1,5,3,4,0]
Output: [-1,0,3,4,5]

Example 3:

Input: head = []
Output: []

Constraints:

  • The number of nodes in the list is in the range [0, 5 * 104].
  • -105 <= Node.val <= 105

Follow up: Can you sort the linked list in O(n logn) time and O(1) memory (i.e. constant space)?

Sort List LeetCode Solution in C++

  /// Merge two sorted lists into one, the head and tail of the new list is returned
  std::pair<ListNode*, ListNode*> merge_lists(ListNode* l1, ListNode* l2)
  {
    ListNode* head;
    ListNode* tail = nullptr;
    ListNode** next_ptr = &head;
    while (l1 || l2) {
      ListNode** list = (!l2 || l1 && l1->val <= l2->val) ? &l1 : &l2;
      *next_ptr = tail = *list;
      *list = (*list)->next;
      next_ptr = &(*next_ptr)->next;
    }
    *next_ptr = nullptr;
    return std::make_pair(head, tail);
  }

  /// Take a list and split it into sublists of the requested size, returns number sublists and remaining list
  std::pair<int, ListNode*> split_list(ListNode* head, int sz, ListNode* lists[], int num_lists)
  {
    int count = 0;
    while (head && count < num_lists) {
      lists[count++] = head;
      ListNode* list_tail = nullptr;
      for (int i = 0; i < sz && head; ++i, head = head->next) {
        list_tail = head;
      }
      if (list_tail) {
        list_tail->next = nullptr;
      }
    }
    return std::make_pair(count, head);
  }
  
  ListNode* sortList(ListNode* head)
  {
    // Working buffer size, must be an even number
    constexpr int buf_sz = 8;
    ListNode* buf[buf_sz];

    bool done = (!head);
    // Initially split list into sublists of size 1, then increase the size until
    // the entire list is sorted
    for (int step = 1; !done; step *= buf_sz) {
      done = true;
      int num;
      ListNode* remaining = head;
      ListNode* tail = nullptr;
      ListNode** next_ptr = &head;
      do {
        // Split the list into increasingly larger sublists
        std::tie(num, remaining) = split_list(remaining, step, buf, buf_sz);
      
        // We'll be done if this is the first split this pass and there is no more remaining
        done &= (!remaining);
        
        // Keep merging our sublists together until a single sorted list is made
        for (; 1 < num; num = (num + 1) / 2) {
          // Merge each pair of sublists
          for (int i = 0; i < num / 2; ++i) {
            std::tie(buf[i], tail) = merge_lists(buf[i * 2], buf[i * 2 + 1]);
          }
          
          // If there is an odd number of lists, just move the last one. It will get
          // merged next time around
          if (num % 2) {
            buf[num / 2] = buf[num - 1];
          }
        }
        *next_ptr = buf[0];
        next_ptr = &(tail->next);
      } while (remaining);
    }

    return head;
  }

Sort List LeetCode Solution in Java

  public ListNode sortList(ListNode head) {
    ListNode dummy = new ListNode(0);
    dummy.next = head;
    ListNode [] list = new ListNode[2];
    boolean done = (null == head);
    // Keep partitioning our list into bigger sublists length. Starting with a size of 1 and doubling each time
    for (int step = 1; !done; step *= 2) {
      done = true;
      ListNode prev = dummy;
      ListNode remaining = prev.next;
      do {
        // Split off two sublists of size step
        for (int i = 0; i < 2; ++i) {
          list[i] = remaining;
          ListNode tail = null;
          for (int j = 0; j < step && null != remaining; ++j, remaining = remaining.next) {
            tail = remaining;
          }
          // Terminate our sublist
          if (null != tail) {
            tail.next = null;
          }
        }

        // We're done if these are the first two sublists in this pass and they
        // encompass the entire primary list
        done &= (null == remaining);

        // If we have two sublists, merge them into one
        if (null != list[1]) {
          while (null != list[0] || null != list[1]) {
            int idx = (null == list[1] || null != list[0] && list[0].val <= list[1].val) ? 0 : 1;
            prev.next = list[idx];
            list[idx] = list[idx].next;
            prev = prev.next;
          }

          // Terminate our new sublist
          prev.next = null;
        } else {
          // Only a single sublist, no need to merge, just attach to the end
          prev.next = list[0];
        }
      } while (null != remaining);
    }
    return dummy.next;
  }

Sort List LeetCode Solution in Python

class Solution(object):
    def merge(self, h1, h2):
        dummy = tail = ListNode(None)
        while h1 and h2:
            if h1.val < h2.val:
                tail.next, tail, h1 = h1, h1, h1.next
            else:
                tail.next, tail, h2 = h2, h2, h2.next
    
        tail.next = h1 or h2
        return dummy.next
    
    def sortList(self, head):
        if not head or not head.next:
            return head
    
        pre, slow, fast = None, head, head
        while fast and fast.next:
            pre, slow, fast = slow, slow.next, fast.next.next
        pre.next = None

        return self.merge(*map(self.sortList, (head, slow)))
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