BST树的所有可能情况

Given n, how many structurally unique BST’s (binary search trees) that store values 1…n?

For example,
Given n = 3, there are a total of 5 unique BST’s.

int numTrees(int n) {
    if(n==0)
        return 1;
    if(n==1)
        return 1;
    if(n%2==0){
        int temp=0;
        for(int i=0;i<n/2;i++){
          temp=2*(numTrees(i)*numTrees(n-1-i))+temp; 
        }
        return temp;
    }
    if(n%2==1){
        int temp=0;
        for(int i=0;i<(n-1)/2;i++){
          temp=2*(numTrees(i)*numTrees(n-1-i))+temp; 
        }
        temp=temp+numTrees((n-1)/2)*numTrees((n-1)/2);
        return temp;
      }
}