算法通关村第二关|白银|链表反转拓展【持续更新】

1.指定区间反转

1.1 头插法：将区间内遍历到的结点插入到起始处之前。

public ListNode reverseBetween(ListNode head, int left, int right) {ListNode dummyNode = new ListNode(-1);dummyNode.next = head;ListNode pre = dummyNode;// 将pre移动到区间的前一位，pre.next指向每次遍历到的需要插入到起始处的结点for (int i = 0; i < left - 1; i++) {pre = pre.next;}// cur负责遍历ListNode cur = pre.next;// next存储遍历到的结点ListNode next;for (int i = 0; i < right - left; i++) {//存储遍历到的结点cur.nextnext = cur.next;//将cur.next向后移动，下次循环继续遍历cur.next = next.next;//让遍历到的结点指向起始处的结点next.next = pre.next;//pre指向新的起始处pre.next = next;}return dummyNode.next;
}

1.2 穿针引线法：找到需要裁切的地方，将子链表整体反转，然后再缝到切开的地方。

public ListNode reverseBetween(ListNode head, int left, int right){ListNode dummyNode = new ListNode(-1);dummyNode.next = head;ListNode pre = dummyNode;// 找 left 结点的前一个结点for (int i = 0; i < left - 1; i++) {pre = pre.next;}// 找 right 结点ListNode rightNode = pre;for (int i = 0; i < right - left + 1; i++) {rightNode = rightNode.next;}// 切割子链表ListNode leftNode = pre.next;ListNode succ = rightNode.next;rightNode.next = null;// 反转链表reverseList(leftNode);// 拼接pre.next = rightNode;leftNode.next = succ;return dummyNode.next;
}
// 反转链表算法
public ListNode reverseList(ListNode head) {ListNode prev = null;ListNode curr = head;while(curr != null){ListNode next = curr.next;// 这里对链表的结构进行了更改// 所以不需要返回值，但是上面的leftNode结构已经更改了curr.next = prev;prev = curr;curr = next;}return prev;}
}

2.两两交换链表中的节点

捋清楚成对交换结点时的指针指向即可。

public ListNode swapPairs(ListNode head) {ListNode dummyHead = new ListNode(0);dummyHead.next = head;ListNode temp = dummyHead;while (temp.next != null && temp.next.next != null) {ListNode node1 = temp.next;ListNode node2 = temp.next.next;temp.next = node2;node1.next = node2.next;node2.next = node1;temp = node1;}return dummyHead.next;
}

3.单链表加1

3.1 基于栈实现。

public ListNode plusOne(ListNode head) {Stack<Integer> stack = new Stack();while (head != null) {stack.push(head.val);head = head.next;}int carry = 0;ListNode dummy = new ListNode(0);// 本题要加1，所以设置了adder为1int adder = 1;while (!stack.empty() || carry > 0) {int digit = stack.empty() ? 0 : stack.pop();int sum = digit + carry + adder;carry = sum / 10;sum = sum % 10;ListNode cur = new ListNode(sum);cur.next = dummy.next;dummy.next = cur;//加一次以后adder就可以置0了adder = 0;}return dummy.next;
}

3.2 链表反转实现。【持续更新】

4.链表加法

4.1 栈实现：栈顶都是两个数的最低位，所以可以一起弹出。

public ListNode addInListByStack(ListNode head1, listNode head2) {Stack<ListNode> stack1 = new Stack<>();Stack<ListNode> stack2 = new Stack<>();while (head1 != null) {stack1.push(head1);head1 = head1.next;}while (head2 != null) {stack2.push(head2);head2 = head2.next;}ListNode newHead = new ListNode(-1);int carry = 0;while (!stack1.empty() || !stack2.empty() || carry != 0) {ListNode a = new ListNode(0);ListNode b = new ListNode(0);if (!stack1.empty()) {a = stack1.pop();}if (!stack2.empty()) {b = stack2.pop();}int get_sum = a.val + b.val + carry;int ans = get_sum % 10;carry = get_sum / 10;ListNode cur = new ListNode(ans);cur.next = newHead.next;newHead.next = cur;}return newHead.next;
}

4.2 链表反转实现。

public class Solution {public ListNode addInList(ListNode head1, ListNode head2) {head1 = reverse(head1);head2 = reverse(head2);ListNode head = new ListNode(-1);ListNode cur = head;int carry = 0;while (head1 != null || head2 != null || carry != 0) {int sum = carry;if (head1 != null) {sum += head1.val;head1 = head1.next;}if (head2 != null) {sum != head2.val;head2 = head2.next;}cur.next = new ListNode(sum % 10);carry = sum / 10;cur = cur.next;}return reverse(head.next);}// 链表反转方法public ListNode reverse(ListNode head){ListNode cur = head;ListNode pre = null;while (cur != null) {ListNode temp = cur.next;cur.next = pre;pre = cur;cur = temp;}return pre;}
}

4.3 链表减法：【持续更新】。

5.再论链表的回文序列问题

书接上回，可以使用栈或者链表反转来做。

public boolean isPalindrome(ListNode head) {if (head == null || head.next == null) {return true;}ListNode slow = head, fast = head;ListNode pre = head, prepre = null;while (fast != null && fast.next != null) {pre = slow;slow = slow.next;fast = fast.next.next;pre.next = prepre;prepre = pre;}if (fast != null) {slow = slow.next;}while (pre != null && slow != null) {if (pre.val != slow.val) {return false;}pre = pre.next;slow = slow.next;}return true;
}

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