61. Rotate List

Brute force

• Pick up the last node and put it to the first, do this k times.
• TC: O(k*N)
• SC: O(1)

Optimized

• Get the length of the linked list.
• point last nodes next to head.
• point the len-kth nodes next to null
• TC: O(N)
• SC: O(1)

Code

class Solution {
public:
    ListNode* rotateRight(ListNode* head, int k) {
        // edge cases
        if (!head || !head->next || k == 0) return head;

        // compute the length
        ListNode *cur = head;
        int len = 1;
        while (cur->next && ++len)
            cur = cur->next;

        // connect last node to head
        cur->next = head;

        // go till that node
        k = k % len;
        k = len - k;
        while (k--) cur = cur->next;

        // make the node head and break connection
        head = cur->next;
        cur->next = NULL;

        return head;
    }
};