Iterative 16ms C solution; 11 lines with comments

  • 0
    typedef struct ListNode ln;
    void reorderList(ln* head) {
      if(!head) return;
      int len = 0, idx = 0;         /* idx is the index of the stack */
      for(ln* tmp = head; tmp; tmp=tmp->next)   len++;
      ln *stack[len], *p;
      /* push all node pointers into the stack */
      for(p = head; p; p=p->next) stack[idx++] = p; 
      for(--idx; head != stack[idx] && head->next != stack[idx]; idx--)
        /* the head of the list points to the top of the stack (i.e. the tail of the list) */
        p = head->next;
        head->next = stack[idx];
        head->next->next = p;
        head = p;
      /* termintate the linked list */
      (stack[idx])->next = NULL;

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    You stored some nodes, so I dont think it is an in-place algorithm

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    @WTCCTW, thanks for your comments. The problem states: "You must do this in-place without altering the nodes' values." I think my solution can still be considered as in-place for two reasons. 1) I am not altering the value of any nodes. 2) The space complexity is much less than O(n), although not O(1), because I am not storing any node, I am storing the POINTERs (in a stack) to nodes, which usually have much smaller size than actually node struct in real world problems. Please let me know if my argument makes sense or not.

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