LeetCode 讀書會第 19 次聚會 2021/01/12

紀錄/LeetCode 讀書會第 19 次聚會 2021_01_12.md

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+# LeetCode 讀書會第 19 次聚會 2021/01/12
+
+leetcode 讀書會通知
+1. 項目: 第 19 次聚會
+2. 目的: 線上一起寫題目, 由有想法的人帶領, 先解題, 再看該題有趣的解法
+3. 時間: 01/12 (二) 20:00 ~ 21:00
+4. 地點: google meet 線上 (前 10 分鐘預備鏈接)
+5. 解題項目:  [Linked list - classic problem 的最後一題 Remove Nth Node From End Of List](https://leetcode.com/explore/learn/card/linked-list/219/classic-problems/)
+
+6. 共筆: GitHub https://github.com/programmingbookclub/Leetcode-club
+
+通知 9 人: @游諭, @turtle, @james, @Louis, @TzuHsuan
+
+備註: 這次的題目是：
+
+
+1. 203 - easy： Remove Linked List Elements
+2. 328 - Medium： Odd Even Linked List
+3. 234 - easy：Palindrome Linked List
+
+
+請在 issue 回報一下，我會把你加入 contribution
+
+https://zh.wikipedia.org/wiki/差一错误
+
+# Remove Linked List Elements
+
+```swift
+// swift
+ func removeElements(_ head: ListNode?, _ val: Int) -> ListNode? {
+    let dummy = ListNode(0, head)
+    var prev: ListNode? = dummy
+    var current: ListNode? = head
+
+    while(current != nil) {
+        if current?.val == val { // 檢查 val 
+                prev?.next = current?.next 
+        } else {
+            prev = current // 更新 prev
+        }
+        current = current?.next // 往後走
+    }
+    return dummy.next
+}
+```
+
+```swift
+// swift
+/**
+ * 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 removeElements(_ head: ListNode?, _ val: Int) -> ListNode? {
+        let dummy = ListNode(val &+ 1, head)
+        var current: ListNode? = dummy
+
+        while(current != nil) {
+
+            if current?.next?.val == val { // 檢查 val 
+                    current?.next = current?.next?.next
+            } else {
+                    current = current?.next // 往後走
+            }
+        }
+        return dummy.next
+    }
+}
+```
+
+# Odd Even Linked List
+
+
+# Palindrome Linked List
+
+## Recursive
+
+```swift=
+// swift
+/**
+ * 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 {
+    /*
+
+    1
+    11
+    121  2 11
+    1221 22 11
+    1231 23 11
+    */
+    func isPalindrome(_ head: ListNode?) -> Bool {
+    var ptr: ListNode? = head
+        func helper(_ head: ListNode?) -> Bool {
+            if head == nil {return true}
+            if helper(head?.next) == false {return false}
+            if (head?.val != ptr?.val) {return false}
+            ptr = ptr?.next
+            return true
+        }
+
+    return helper(head)
+    }
+}
+```
+
+## Iterative & in-place
+
+遍歷整個 List 3 次
+1. 計算長度
+2. 反轉後半段
+3. Two pointer 從兩端檢查是否相同
+
+```typescript=
+// TypeScript
+
+/** 
+    Iterative & in-place
+    time: O(3n) = O(n), space: O(1)
+*/
+function isPalindrome(head: ListNode | null): boolean {
+    // 算出長度
+    const length = getListLength(head);
+    if (length <= 1) return true;
+    // 反轉後半段，並額外考慮長度為偶數的情況
+    const middleNode = getNthNode(head, Math.ceil(length / 2));
+    const anotherHead = length % 2 === 0 ? reverseList(middleNode.next) : reverseList(middleNode);
+    middleNode.next = null;
+    // Two pointer 從兩端檢查是否相同
+    return isSameList(head, anotherHead);
+};
+
+// 以下為輔助函式
+function isSameList(headA: ListNode, headB: ListNode): boolean {
+    let currA = headA;
+    let currB = headB;
+    while(currA != null) {
+        if (currA.val !== currB.val) return false;
+        currA = currA.next;
+        currB = currB.next;
+    }
+    if (currB == null) return true;
+    else return false;
+}
+
+function getListLength(head: ListNode): number {
+    let count = 0;
+    let curr = head;
+    while(curr != null) {
+        count++;
+        curr = curr.next;
+    }
+    return count;
+}
+
+function getNthNode(head: ListNode, n: number): ListNode {
+    if (head == null || !Number.isInteger(n) || n < 1) return null;
+    let count = 1;
+    let curr = head;
+    while(count < n) {
+        curr = curr?.next;
+        count++;
+    }
+    return curr;
+}
+
+/** Iteratively */
+function reverseList(head: ListNode | null): ListNode | null {
+    if (!head) return null;
+    let curr = head;
+    let prev: ListNode = null;
+    let next: ListNode = null;
+    while(curr) {
+        next = curr.next ?? null;
+        curr.next = prev;
+        prev = curr;
+        curr = next;
+    }
+    return prev;
+};
+```
+
+
+
+![](https://i.imgur.com/UJSIGtG.png)
+
+比較了 Recursive vs Iterative
+
+Runtime: 92 vs 100 ms
+
+Memory: 45.7 vs 42.3 MB
+