g0201_0300.s0206_reverse_linked_list.Solution.swift Maven / Gradle / Ivy

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104 LeetCode algorithm problem solutions

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// #Easy #Top_100_Liked_Questions #Top_Interview_Questions #Linked_List #Recursion
// #Data_Structure_I_Day_8_Linked_List #Algorithm_I_Day_10_Recursion_Backtracking
// #Level_1_Day_3_Linked_List #Udemy_Linked_List #Big_O_Time_O(N)_Space_O(1)
// #2024_06_28_Time_4_ms_(89.47%)_Space_16_MB_(24.52%)

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public var val: Int
 *     public var next: ListNode?
 *     public init() { self.val = 0; self.next = nil; }
 *     public init(_ val: Int) { self.val = val; self.next = nil; }
 *     public init(_ val: Int, _ next: ListNode?) { self.val = val; self.next = next; }
 * }
 */
class Solution {
    func reverseList(_ head: ListNode?) -> ListNode? {
        var prev: ListNode? = nil
        var curr = head
        var next = head?.next

        while curr != nil {
            // reverse the current nodes link
            curr?.next = prev
            
            // update variables
            prev = curr
            curr = next
            next = next?.next
        }
        return prev
    }
}