Java LeetCode 96. Different binary search trees

Given an integer n, how many kinds of binary search trees with 1… n as nodes?

Here, map is used to cache the traversal times of each n, which greatly reduces the number of constructions and speeds up time.

class Solution {
    
    
    Map<Integer,Integer> map = new HashMap();
    public int numTrees(int n) {
    
    
       
        int sum=0;
        if(n<=1){
    
    
            map.put(n,1);
            return 1;
        }
        if(map.containsKey(n)){
    
    
            return map.get(n);
        }
        for(int i=1;i<=n;i++){
    
    
            sum += numTrees(i-1)*numTrees(n-i);
            
        }
        map.put(n,sum);
        return sum;
    }
}