foreword

I only record my experience in preparing Leetcode. You can refer to most of the questions you have done. There are
22 questions in total.

1. leetcode 45. Jumping Game II -java version

2. Leetcode 101. Symmetric binary tree-java implementation

Original link

3. 349. Intersection of two arrays

4. Leetcode 205. Isomorphic string-java implementation

I didn't understand this question at first

5. Leetcode Weekly Competition 2386. Find the Kth largest sum of an array - java implementation

This question is the most difficult to understand

6. Leetcode 102. Level order traversal of binary tree - java implementation

Seven, leetcode 1669. Merge two linked lists - java implementation

8. Sword Pointer Offer II 074. Merge interval

The array intervals represents a collection of several intervals, and a single interval is intervals[i] = [starti, endi]. Please merge all overlapping intervals and return an array of non-overlapping intervals that exactly covers all intervals in the input.

Input: intervals = [[1,3],[2,6],[8,10],[15,18]]
Output: [[1,6],[8,10],[15,18]]
Explanation: The intervals [1,3] and [2,6] overlap, merge them into [1,6].

This question has been similarly sorted by numbers first and then divided into two situations, and finally remember to count the last group

class Solution {
    
    
    public int[][] merge(int[][] a) {
    
    
       // Arrays.sort(intervals,(o1,o2)->(o1[0] == o2[0] ? o1[1]-o2[1]:o1[0]-o2[0]));
         List<int[]> res = new ArrayList<>();
           if(a.length ==0 ) return null;
           Arrays.sort(a, (i1, i2) -> (i1[0] == i2[0] ? i2[1] - i1[1] : i1[0] - i2[0]));
           int l = a[0][0] ;int r  = a[0][1];//第一个区间的左右端点
           //从第二个端点开始遍历
           for(int i = 1 ; i < a.length ; i++){
    
    
               if(a[i][0] > r ){
    
    
                   res.add(new int[]{
    
    l,r});
                   l = a[i][0];
                   r = a[i][1];
               }else r = Math.max(r,a[i][1]);
           }
           //最后一个区间要保存
           res.add(new int[]{
    
    l,r});
           return res.toArray(new int[0][0]) ;
 
    }
}

Nine, leetcode 54. Spiral matrix-java implementation

leetcode 59. Spiral matrix II-java implementation

10. Sword Pointer Offer II 056. The sum of two nodes in the binary search tree

Given the root node root of a binary search tree and an integer k, please judge whether there are two nodes whose sum is equal to k in the binary search tree. Assume that the values of the nodes in the binary search tree are all unique.

Input: root = [8,6,10,5,7,9,11], k = 12
Output: true
Explanation: The sum of node 5 and node 7 is equal to 12

/**
 * 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 {
    
    
    Set<Integer> s = new HashSet<>();
    public boolean findTarget(TreeNode root, int k) {
    
    
        if(root == null) return false ;
         if(s.contains(k - root.val)) return true;
        s.add(root.val);
        return findTarget(root.left,k)||findTarget(root.right,k);
    }
}