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Sort List in C++
Suppose we have a list, we have to sort this in O(n logn) time using constant space complexity, so if the list is like [4,2,1,3], then it will be [1,2,3,4]
To solve this, we will follow these steps −
Define a method for merging two lists in sorted, order, that method is merge(), this takes two lists l1 and l2.
The sort list method will work like below −
if head is null, or next of head is null, then return head
slow := head, fast := head, and prev = null
while fast is not null and next of fast is also not null, do
prev := slow
slow := next of slow
fast := next of next of fast
next of prev := null
l1 := sortList(head)
l2 := sortList(slow)
return merge(l1, l2)
Let us see the following implementation to get better understanding −
Example
#include <bits/stdc++.h>
using namespace std;
class ListNode{
public:
int val;
ListNode *next;
ListNode(int data){
val = data;
next = NULL;
}
};
ListNode *make_list(vector<int> v){
ListNode *head = new ListNode(v[0]);
for(int i = 1; i<v.size(); i++){
ListNode *ptr = head;
while(ptr->next != NULL){
ptr = ptr->next;
}
ptr->next = new ListNode(v[i]);
}
return head;
}
void print_list(ListNode *head){
ListNode *ptr = head;
cout << "[";
while(ptr){
cout << ptr->val << ", ";
ptr = ptr->next;
}
cout << "]" << endl;
}
class Solution {
public:
ListNode* sortList(ListNode* head) {
if(!head || !head->next)return head;
ListNode *slow = head, *fast = head, *prev = NULL;
while(fast && fast->next){
prev = slow;
slow = slow->next;
fast = fast->next->next;
}
prev->next = NULL;
ListNode* l1 = sortList(head);
ListNode* l2 = sortList(slow);
return mergeList(l1,l2);
}
ListNode* mergeList(ListNode* l1, ListNode* l2){
ListNode* temp = new ListNode(0);
ListNode* p =temp;
while(l1 && l2){
if(l1->val<=l2->val){
p->next = l1;
l1 = l1->next;
}else{
p->next = l2;
l2 = l2->next;
}
p = p->next;
}
if(l1){
p->next = l1;
}
if(l2){
p->next = l2;
}
return temp->next;
}
};
main(){
vector<int> v = {4,2,1,3,5,19,18,6,7};
ListNode *h1 = make_list(v);
Solution ob;
print_list((ob.sortList(h1)));
}Input
[4,2,1,3,5,9,8,6,7]
Output
[1, 2, 3, 4, 5, 6, 7, 18, 19, ]
Updated on: 30-Apr-2020
184 Views
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