g0001_0100.s0025_reverse_nodes_in_k_group.Solution.rs Maven / Gradle / Ivy

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104 LeetCode algorithm problem solutions

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// #Hard #Top_100_Liked_Questions #Linked_List #Recursion #Data_Structure_II_Day_13_Linked_List
// #Udemy_Linked_List #Big_O_Time_O(n)_Space_O(k)
// #2024_09_05_Time_0_ms_(100.00%)_Space_2.3_MB_(100.00%)

// Definition for singly-linked list.
// #[derive(PartialEq, Eq, Clone, Debug)]
// pub struct ListNode {
//   pub val: i32,
//   pub next: Option>
// }
// 
// impl ListNode {
//   #[inline]
//   fn new(val: i32) -> Self {
//     ListNode {
//       next: None,
//       val
//     }
//   }
// }
impl Solution {
    pub fn reverse_k_group(mut head: Option>, k: i32) -> Option> {
        Solution::reverse(head, k)
    }
    pub fn reverse(mut head:Option>, k: i32) -> Option> {
        if head.is_none() {
            return None;
        }
        let mut new_head: Option> = None;
        let mut i = 0;
        while let Some(mut head_val) = head.take() {
            head = head_val.next.take();
            head_val.next = new_head;
            new_head = Some(head_val);
            i += 1;
            if i == k {
                break;
            }
        }
        if i != k {
            // have to back out and return original list
            return Solution::reverse(new_head, i);
        }
        // we now have two lists:
        // head -> rest of list
        // new_head -> reversed list
        // i cannot figure out how in rust you do this in one pass, 
        // so we will go walk the list again to get the tail of new_head and make it head
        // 'append'
        head = Solution::reverse(head, k);
        let mut tailw = &mut new_head;
        if let Some(ref mut tail_val) = tailw {
            let mut tail = tail_val;
            while let Some(ref mut tail_next) = tail.next {
                tail = tail_next;
            }
            tail.next = head;
        }
        new_head
    }
}