Leetcode96_unique Binary Search Trees (1 to n how many different two-fork lookup trees can be formed by these nodes) Java troubleshooting

Topic:

Given N, how many structurally unique BST 's (binary search trees) that store values 1 ... n?

For example,
Given N = 3, there is a total of 5 unique BST ' s.

   1         3     3      2      1    \//      /\           3     2     1      1   3      2    /     /       \                    2     1         2                 3

Solving:

With recursive thinking, when there are only 0 or 1 nodes. There is only one. N nodes have an F (n) Type:

The left side can have n-1 nodes, the right 0 nodes, according to the symmetry can be interchanged around. At this time there are 2*f (n-1) *f (0);

On one side 1, there is a side n-2, this time has 2*f (1) *f (n-2);

One side two, one side N-3, this time has 2*f (2) *f (n-3);

。。。

。。。

Suppose N is an odd number and both sides are N/2. At this time there is f (N/2) *f (N/2), assuming that n is even, one side N/2 side (n/2+1), for 2*f (N/2) *f (n/2+1);

Code:

  public static int numtrees (int n) {     if (n==1| | n==0)     return 1;     int sum=0;     if (n%2==0)     {for     (int k=n-1;k>=n/2;k--)         {         sum+=2*numtrees (k) *numtrees (n-1-k);         }         return sum;          }     else {for     (int k=n-1;k>n/2;k--)         {         sum+=2*numtrees (k) *numtrees (n-1-k);         }         Return Sum+numtrees (N/2) *numtrees (N/2);}            }

Leetcode96_unique Binary Search Trees (1 to n how many different two-fork lookup trees can be formed by these nodes) Java troubleshooting