Sort List Problem


Description

LeetCode Problem 148.

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

Example 1:

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

Example 2:

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

Example 3:

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Input: head = []
Output: []

Constraints:

  • The number of nodes in the list is in the range [0, 5 * 10^4].
  • -10^5 <= Node.val <= 10^5


Sample C++ Code

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class Solution {
public:
	ListNode *sortList(ListNode *head) {
		if(!head || !(head->next)) return head;
		
		//get the linked list's length
		ListNode* cur = head;
		int length = 0;
		while(cur){
			length++;
			cur = cur->next;
		}
		
		ListNode dummy(0);
		dummy.next = head;
		ListNode *left, *right, *tail;
		for(int step = 1; step < length; step <<= 1){
			cur = dummy.next;
			tail = &dummy;
			while(cur){
				left = cur;
				right = split(left, step);
				cur = split(right,step);
				tail = merge(left, right, tail);
			}
		}
		return dummy.next;
	}
	
private: 
	ListNode* split(ListNode *head, int n){
		//if(!head) return NULL;
		for(int i = 1; head && i < n; i++) head = head->next;
		
		if(!head) return NULL;
		ListNode *second = head->next;
		head->next = NULL;
		return second;
	}
	
	ListNode* merge(ListNode* l1, ListNode* l2, ListNode* head){
		ListNode *cur = head;
		while(l1 && l2){
			if(l1->val > l2->val){
				cur->next = l2;
				cur = l2;
				l2 = l2->next;
			}
			else{
				cur->next = l1;
				cur = l1;
				l1 = l1->next;
			}
		}
		cur->next = (l1 ? l1 : l2);
		while(cur->next) cur = cur->next;
		return cur;
	}
};




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