【戊】链表


链表是空节点,或者有一个值和一个指向下一个链表的指针,因此很多链表问题可以用递归来处理。

部分转自:

1. 找出两个链表的交点

160. Intersection of Two Linked Lists (Easy)

Leetcode / 力扣

例如以下示例中 A 和 B 两个链表相交于 c1:

A:          a1 → a2
                    ↘
                      c1 → c2 → c3
                    ↗
B:    b1 → b2 → b3

但是不会出现以下相交的情况,因为每个节点只有一个 next 指针,也就只能有一个后继节点,而以下示例中节点 c 有两个后继节点。

A:          a1 → a2       d1 → d2
                    ↘  ↗
                      c
                    ↗  ↘
B:    b1 → b2 → b3        e1 → e2

要求时间复杂度为 O(N),空间复杂度为 O(1)。如果不存在交点则返回 null。

设 A 的长度为 a + c,B 的长度为 b + c,其中 c 为尾部公共部分长度,可知 a + c + b = b + c + a。

当访问 A 链表的指针访问到链表尾部时,令它从链表 B 的头部开始访问链表 B;同样地,当访问 B 链表的指针访问到链表尾部时,令它从链表 A 的头部开始访问链表 A。这样就能控制访问 A 和 B 两个链表的指针能同时访问到交点。

如果不存在交点,那么 a + b = b + a,以下实现代码中 l1 和 l2 会同时为 null,从而退出循环。

public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
    ListNode l1 = headA, l2 = headB;
    while (l1 != l2) {
        l1 = (l1 == null) ? headB : l1.next;
        l2 = (l2 == null) ? headA : l2.next;
    }
    return l1;
}

如果只是判断是否存在交点,那么就是另一个问题,即 编程之美 3.6 的问题。有两种解法:

  • 把第一个链表的结尾连接到第二个链表的开头,看第二个链表是否存在环;
  • 或者直接比较两个链表的最后一个节点是否相同。

1.1判断链表是否有环

有的问题的隐藏问题其实是判断链表是否有环,比如

202. 快乐数

使用hash或快慢指针可以判断

class Solution {
    public boolean isHappy(int n) {
        int fast = getNext(n);
        int slow = n;
        while(fast != slow && fast != 1){
            slow = getNext(slow);
            fast = getNext(fast);
            fast = getNext(fast);
        }
        return fast==1;
    }

    private int getNext(int num) {
        int res = 0;
        while(num != 0){
            res += (num % 10) * (num % 10);
            num /= 10;
        }
        return res;
    }
}

hash

def isHappy(self, n: int) -> bool:

    def get_next(n):
        total_sum = 0
        while n > 0:
            n, digit = divmod(n, 10)
            total_sum += digit ** 2
        return total_sum

    seen = set()
    while n != 1 and n not in seen:
        seen.add(n)
        n = get_next(n)

    return n == 1

作者:LeetCode-Solution
链接:https://leetcode-cn.com/problems/happy-number/solution/kuai-le-shu-by-leetcode-solution/
来源:力扣(LeetCode)
著作权归作者所有。商业转载请联系作者获得授权,非商业转载请注明出处。

2. 链表反转

206. Reverse Linked List (Easy)

Leetcode / 力扣

递归

public ListNode reverseList(ListNode head) {
    if (head == null || head.next == null) {
        return head;
    }
    ListNode next = head.next;
    ListNode newHead = reverseList(next);
    next.next = head;
    head.next = null;
    return newHead;
}

头插法

public ListNode reverseList(ListNode head) {
    ListNode newHead = new ListNode(-1);
    while (head != null) {
        ListNode next = head.next;
        head.next = newHead.next;
        newHead.next = head;
        head = next;
    }
    return newHead.next;
}

3. 归并两个有序的链表

21. Merge Two Sorted Lists (Easy)

Leetcode / 力扣

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    if (l1 == null) return l2;
    if (l2 == null) return l1;
    if (l1.val < l2.val) {
        l1.next = mergeTwoLists(l1.next, l2);
        return l1;
    } else {
        l2.next = mergeTwoLists(l1, l2.next);
        return l2;
    }
}

4. 从有序链表中删除重复节点

83. Remove Duplicates from Sorted List (Easy)

Leetcode / 力扣

Given 1->1->2, return 1->2.
Given 1->1->2->3->3, return 1->2->3.
public ListNode deleteDuplicates(ListNode head) {
    if (head == null || head.next == null) return head;
    head.next = deleteDuplicates(head.next);
    return head.val == head.next.val ? head.next : head;
}

5. 删除链表的倒数第 n 个节点

19. Remove Nth Node From End of List (Medium)

Leetcode / 力扣

Given linked list: 1->2->3->4->5, and n = 2.
After removing the second node from the end, the linked list becomes 1->2->3->5.
public ListNode removeNthFromEnd(ListNode head, int n) {
    ListNode fast = head;
    while (n-- > 0) {
        fast = fast.next;
    }
    if (fast == null) return head.next;
    ListNode slow = head;
    while (fast.next != null) {
        fast = fast.next;
        slow = slow.next;
    }
    slow.next = slow.next.next;
    return head;
}

5.1 删除值为某数的节点

203. 移除链表元素

class Solution {
    public ListNode removeElements(ListNode head, int val) {
        ListNode pre = new ListNode(-1);
        pre.next = head;
        ListNode res = pre;
        while(head != null){
            if(head.val == val){
                pre.next = head.next;
                head = pre.next;
                continue;
            }
            head = head.next;
            pre = pre.next;

        }
        return res.next;
    }
}

6. 交换链表中的相邻结点

24. Swap Nodes in Pairs (Medium)

Leetcode / 力扣

Given 1->2->3->4, you should return the list as 2->1->4->3.

题目要求:不能修改结点的 val 值,O(1) 空间复杂度。

public ListNode swapPairs(ListNode head) {
    ListNode node = new ListNode(-1);
    node.next = head;
    ListNode pre = node;
    while (pre.next != null && pre.next.next != null) {
        ListNode l1 = pre.next, l2 = pre.next.next;
        ListNode next = l2.next;
        l1.next = next;
        l2.next = l1;
        pre.next = l2;

        pre = l1;
    }
    return node.next;
}

7. 链表求和

445. Add Two Numbers II (Medium)

Leetcode / 力扣

Input: (7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 8 -> 0 -> 7

题目要求:不能修改原始链表。

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    Stack<Integer> l1Stack = buildStack(l1);
    Stack<Integer> l2Stack = buildStack(l2);
    ListNode head = new ListNode(-1);
    int carry = 0;
    while (!l1Stack.isEmpty() || !l2Stack.isEmpty() || carry != 0) {
        int x = l1Stack.isEmpty() ? 0 : l1Stack.pop();
        int y = l2Stack.isEmpty() ? 0 : l2Stack.pop();
        int sum = x + y + carry;
        ListNode node = new ListNode(sum % 10);
        node.next = head.next;
        head.next = node;
        carry = sum / 10;
    }
    return head.next;
}

private Stack<Integer> buildStack(ListNode l) {
    Stack<Integer> stack = new Stack<>();
    while (l != null) {
        stack.push(l.val);
        l = l.next;
    }
    return stack;
}

8. 回文链表

234. Palindrome Linked List (Easy)

Leetcode / 力扣

题目要求:以 O(1) 的空间复杂度来求解。

切成两半,把后半段反转,然后比较两半是否相等。

public boolean isPalindrome(ListNode head) {
    if (head == null || head.next == null) return true;
    ListNode slow = head, fast = head.next;
    while (fast != null && fast.next != null) {
        slow = slow.next;
        fast = fast.next.next;
    }
    if (fast != null) slow = slow.next;  // 偶数节点,让 slow 指向下一个节点
    cut(head, slow);                     // 切成两个链表
    return isEqual(head, reverse(slow));
}

private void cut(ListNode head, ListNode cutNode) {
    while (head.next != cutNode) {
        head = head.next;
    }
    head.next = null;
}

private ListNode reverse(ListNode head) {
    ListNode newHead = null;
    while (head != null) {
        ListNode nextNode = head.next;
        head.next = newHead;
        newHead = head;
        head = nextNode;
    }
    return newHead;
}

private boolean isEqual(ListNode l1, ListNode l2) {
    while (l1 != null && l2 != null) {
        if (l1.val != l2.val) return false;
        l1 = l1.next;
        l2 = l2.next;
    }
    return true;
}

9. 分隔链表

725. Split Linked List in Parts(Medium)

Leetcode / 力扣

Input:
root = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], k = 3
Output: [[1, 2, 3, 4], [5, 6, 7], [8, 9, 10]]
Explanation:
The input has been split into consecutive parts with size difference at most 1, and earlier parts are a larger size than the later parts.

题目描述:把链表分隔成 k 部分,每部分的长度都应该尽可能相同,排在前面的长度应该大于等于后面的。

public ListNode[] splitListToParts(ListNode root, int k) {
    int N = 0;
    ListNode cur = root;
    while (cur != null) {
        N++;
        cur = cur.next;
    }
    int mod = N % k;
    int size = N / k;
    ListNode[] ret = new ListNode[k];
    cur = root;
    for (int i = 0; cur != null && i < k; i++) {
        ret[i] = cur;
        int curSize = size + (mod-- > 0 ? 1 : 0);
        for (int j = 0; j < curSize - 1; j++) {
            cur = cur.next;
        }
        ListNode next = cur.next;
        cur.next = null;
        cur = next;
    }
    return ret;
}

10. 链表元素按奇偶聚集

328. Odd Even Linked List (Medium)

Leetcode / 力扣

Example:
Given 1->2->3->4->5->NULL,
return 1->3->5->2->4->NULL.

public ListNode oddEvenList(ListNode head) {
    if (head == null) {
        return head;
    }
    ListNode odd = head, even = head.next, evenHead = even;
    while (even != null && even.next != null) {
        odd.next = odd.next.next;
        odd = odd.next;
        even.next = even.next.next;
        even = even.next;
    }
    odd.next = evenHead;
    return head;
}

奇偶链表

328. 奇偶链表

  • 边界情况的谈论切忌复杂,本题even永远在odd后面,所以只关注这一个即可
public class Solution {
    public ListNode oddEvenList(ListNode head) {
        if (head == null) return null;
        ListNode odd = head, even = head.next, evenHead = even;
        while (even != null && even.next != null) {
            odd.next = even.next;
            odd = odd.next;
            even.next = odd.next;
            even = even.next;
        }
        odd.next = evenHead;
        return head;
    }
}

旋转链表

61. 旋转链表
给定一个链表,旋转链表,将链表每个节点向右移动 k 个位置,其中 k 是非
负数。

  • 使用循环移动法简单
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode rotateRight(ListNode head, int k) {
        if (head == null) return null;
        ListNode prev = head;
        // 记录长度
        int length = 1;
        while (prev.next != null) {
            prev = prev.next;
            length++;
        }
        //将链表变成圈状,方便循环移位,太巧妙了
        prev.next = head;
        // k是可以大于长度的,所以循环取模
        int n = length - k % length;
        while (--n >= 0) {
            prev = prev.next;
        }
        head = prev.next;
        prev.next = null;
        return head;
    }
}

链表合并

合并两个有序链表

可以使用递归或迭代

private ListNode merge2Lists(ListNode l1, ListNode l2) {
    if (l1 == null) {
        return l2;
    }
    if (l2 == null) {
        return l1;
    }
    if (l1.val < l2.val) {
        l1.next = merge2Lists(l1.next, l2);
        return l1;
    }
    l2.next = merge2Lists(l1, l2.next);
    return l2;
}

使用一个哑节点非常方便

private ListNode merge2Lists(ListNode l1, ListNode l2) {
    ListNode dummyHead = new ListNode(0);
    ListNode tail = dummyHead;
    while (l1 != null && l2 != null) {
        if (l1.val < l2.val) {
            tail.next = l1;
            l1 = l1.next;
        } else {
            tail.next = l2;
            l2 = l2.next;
        }
        tail = tail.next;
    }
    tail.next = l1 == null? l2: l1;

    return dummyHead.next;
}

其他

合并k个链表

比较好的方法:
1.两两合并

  1. 迭代
  2. 递归

2.优先级队列

下面是代码
1.1
//方法非常好,从头到尾两两组队,且是原地合并

class Solution {
      public ListNode mergeKLists(ListNode[] lists) {
            if(lists.length < 1) return null;
            //法1:分治
            int k = lists.length;
            //方法非常好,从头到尾两两组队,且是原地合并
            while(k > 1){
                  int cur = 0;
                  for (int i = 0; i < k; i+=2) {
                        if(i==k-1) lists[cur++] = lists[i];
                        else lists[cur++] = merge2Lists(lists[i],lists[i+1]); 
                  }
                  k = cur;
            }
            return lists[0];
      }
      
      private ListNode merge2Lists(ListNode l1, ListNode l2) {
            if (l1 == null) {
                  return l2;
            }
            if (l2 == null) {
                  return l1;
            }
            if (l1.val < l2.val) {
                  l1.next = merge2Lists(l1.next, l2);
                  return l1;
            }
            l2.next = merge2Lists(l1, l2.next);
            return l2;
      }
}

1.2
就是一个简单的归并。

class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if (lists.length == 0) {
            return null;
        }
        return merge(lists, 0, lists.length - 1);
    }

    private ListNode merge(ListNode[] lists, int lo, int hi) {
        if (lo == hi) {
            return lists[lo];
        }
        int mid = lo + (hi - lo) / 2;
        ListNode l1 = merge(lists, lo, mid);
        ListNode l2 = merge(lists, mid + 1, hi);
        return merge2Lists(l1, l2);
    }
}

作者:sweetiee
链接:https://leetcode-cn.com/problems/merge-k-sorted-lists/solution/4-chong-fang-fa-xiang-jie-bi-xu-miao-dong-by-sweet/
来源:力扣(LeetCode)
著作权归作者所有。商业转载请联系作者获得授权,非商业转载请注明出处。

2
特殊情况:双null
lists是个null
lists里面的元素是null

class Solution {
      public ListNode mergeKLists(ListNode[] lists) {
            ListNode d = new ListNode(-1);
            ListNode res = d;
            Queue<ListNode> pq = new PriorityQueue<>((l1,l2)->l1.val-l2.val);
            for (int i = 0; i < lists.length; i++) {
                if(lists[i] != null)  //一定要主意测试用例里面的情况
                    pq.offer(lists[i]);
            }
            while(!pq.isEmpty()){
                  ListNode t =  pq.poll();
                  d.next = t;
                  d = t;
                  if(t.next != null){
                        pq.offer(t.next);
                  }
            }
            return res.next;
      }
}

反转一个链表

206. 反转链表
递归:
其实挺难想的,每次递归调用当前节点的next后,返回的是翻转后的头节点,所以要将这个头节点慢慢传递到第一层。
在当前层需要做的一个工作是:倒转,即将当前的next的next变成当前的,然后将当前的next置为null。
当前层返回到上一层的是一个以最终返回值为头,当前值为尾的链表。

class Solution {
    public ListNode reverseList(ListNode head) {
        if(head == null ||head.next == null) return head;
        ListNode temp = reverseList(head.next);                     
        head.next.next = head;  
        head.next = null;  
        return temp;
    }
}

迭代:
一开始的pre是翻转后链表的尾节点,所以是null.

class Solution {
    public ListNode reverseList(ListNode head) {
    //迭代
        ListNode pre = null;
        while (head != null){
            ListNode temp = head.next;
            head.next = pre;
            pre = head;
            head = temp;
        }
        return pre;
    }
}

判断是否是回文链表

234. 回文链表
法1:求出一个翻转后的链表:两两比较

https://leetcode-cn.com/problems/palindrome-linked-list/solution/234java-shuang-zhi-zhen-tu-jie-by-ustcyyw/

   class Solution {
    public boolean isPalindrome(ListNode head) {
        ListNode r = reverse(head);
        while(head != null){
            if(head.val != r.val) return false;
            head = head.next;
            r = r.next;
        }
        return true;
    }

    private ListNode reverse(ListNode head) {
        ListNode pre = null;
        ListNode newHead = null;
        while (head != null){
            newHead = new ListNode(head.val);            
            newHead.next = pre;
            pre = newHead;
            head = head.next;
        }
        return newHead;
    }
}

法2:
将链表转化成一个数组,然后按照数组的方式判断。双指针法。

法3 将链表的前面翻转,然后与后面的判断
寻找中间节点的方法:使用快慢指针,所以可以在此过程中直接将前半部分翻转。

class Solution {
    public boolean isPalindrome(ListNode head) {
        if (head == null || head.next == null)
            return true;
        ListNode fast = head;
        ListNode slow = head;
        ListNode pre = null;
        ListNode temp = null; 
        while (fast != null && fast.next != null){
            fast = fast.next.next;
            temp = slow.next;
            slow.next = pre;
            pre = slow;
            slow = temp;
        }
        //此时pre是翻转后的头节点,
        
        //一下是节点个数为奇数的情况
        if(fast != null){
            slow = slow.next;
        }
        while(pre != null){
            if(pre.val != slow.val){
                return false;
            }
            pre = pre.next;
            slow = slow.next;
        }
        return true;
    }
}
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