Unique Binary Search Trees

class Solution {public:    int numTrees(int n) {if(!n) return 0;if(n==1) return 1;if(n==2) return 2;if(n==3) return 5;int *count=new int[n+1];memset(count,0,sizeof(int)*(n+1));count[0]=1,count[1]=1,count[2]=2,count[3]=5;for(int i=4;i<=n;i++){int sum=0;for(int j=1;j<=i;j++)//j表示根节点{sum=sum+count[j-1]*count[i-j];//count[j-1]表示左子树的种类//count[i-j]表示右子树的种类}count[i]=sum;}return count[n];    }};