1. 删除排序链表中的重复元素： https://leetcode.cn/problems/remove-duplicates-from-sorted-list/

2. 合并俩个有序数组： https://leetcode.cn/problems/merge-sorted-array/

   int[]数组转 List: Arrays.stream(arr).collect(Collectors.toList());
   list排序 Collections.sort(list);

import java.util.Arrays;
import java.util.stream.Collectors;class Solution {public void merge(int[] nums1, int m, int[] nums2, int n) {List<Integer> list = new ArrayList<>();list.addAll(Arrays.stream(nums2).boxed().collect(Collectors.toList()));list.addAll(Arrays.stream(nums1).boxed().collect(Collectors.toList()).subList(0, m));Collections.sort(list);for (int i = 0; i < list.size(); i++) {nums1[i] = list.get(i);}}
}

3. 对称二叉树： https://leetcode.cn/problems/symmetric-tree/

给定根节点，依次遍历当前根节点左右节点值是否相同，然后左子树的左节点与右子树的右节点是否相同；左子树的右节点与右子树的子节点是否相同；

/*** Definition for a binary tree node.* public class TreeNode {* int val;* TreeNode left;* TreeNode right;* TreeNode() {}* TreeNode(int val) { this.val = val; }* TreeNode(int val, TreeNode left, TreeNode right) {* this.val = val;* this.left = left;* this.right = right;* }* }*/
class Solution {public boolean isSymmetric(TreeNode root) {if (root == null) {return true;}if (root.left == null && root.right == null) {return true;} else if (root.left == null || root.right == null) {return false;}return isSymmetric2(root.left, root.right);}public boolean isSymmetric2(TreeNode nodeA, TreeNode nodeB) {if (nodeA == null && nodeB == null) {return true;} else if (nodeA == null || nodeB == null) {return false;}if (nodeA.val != nodeB.val) {return false;}if (isSymmetric2(nodeA.left, nodeB.right)) {return isSymmetric2(nodeA.right, nodeB.left);} else {return false;}}
}

4. 验证回文串： https://leetcode.cn/problems/valid-palindrome/

5. 环形链表： https://leetcode.cn/problems/linked-list-cycle/
   法一： 定义一个hashSet，把走过的每个node记录下，下次在经过就认为有环；
   法二： 定义兔子一下走俩步，乌龟一下走一步，看是否乌龟能追到兔子，能追到则认为有环，没追到则认为没有；
   只是判断值，不太对，也有可能不同node有相同的val；

/*** Definition for singly-linked list.* class ListNode {* int val;* ListNode next;* ListNode(int x) {* val = x;* next = null;* }* }*/
public class Solution {public boolean hasCycle(ListNode head) {//List<Integer> list = new ArrayList<>();//return calcCycle(head, list);Set<ListNode> set = new HashSet<>();while(head != null){if(set.contains(head)){return true;}set.add(head);head = head.next;}return false;}public boolean calcCycle(ListNode head, List<Integer> list) {if (head == null) {return false;}if (list != null && list.contains(head.val)) {return true;}list.add(head.val);return calcCycle(head.next, list);}}