Sort List

Sort a linked list in O(n log n) time using constant space complexity.

Code:

class Solution {
public:
    ListNode *sortList(ListNode *head) {
        if(!head) return head;
        map<int,vector<ListNode *>> sortMap;
        vector<int> sortVector;
        ListNode *anchor=new ListNode(0);
        anchor->next=head;
        ListNode *cur=head;
        int n=0;
        ///////////////// put into map and vector /////////////////
        while(cur){
            n++;
            vector<ListNode *> sameValue;
            sortVector.push_back(cur->val);
            sortMap[cur->val].push_back(cur);
            cur=cur->next;
        }
        sort(sortVector.begin(),sortVector.end()); // quick-sort costs O(nlogn)
        ///////////////// reconstruct /////////////////
        anchor->next=sortMap[sortVector[0]].front();
        for(int j=1,len=sortMap[sortVector[0]].size();j<len;j++)
            sortMap[sortVector[0]][j-1]->next=sortMap[sortVector[0]][j];
        for(int i=1;i<n;i++){
            if(sortVector[i-1]!=sortVector[i]){
                sortMap[sortVector[i-1]].back()->next=sortMap[sortVector[i]].front();
                sortMap.erase(sortVector[i-1]);
                for(int j=1,len=sortMap[sortVector[i]].size();j<len;j++)
                    sortMap[sortVector[i]][j-1]->next=sortMap[sortVector[i]][j];
            }
        }
        sortMap[sortVector[n-1]].back()->next=NULL;
        sortMap.erase(sortVector[n-1]);
        //////////////////////////////////////////////
        head=anchor->next;
        delete anchor;
        return head;
    }
};