LeetCode 96.Unique Binary Search Trees

代碼如下：

public class Solution {
   static int[] src;
    public int numTrees(int n) {
        int max = Math.max(n+1,4);
        src = new int[max];
        src[0] = 1;
        src[1] = 1;
        src[2] = 2;
        src[3] = 5;
        return count(n);
    }
    public int count(int n){
        if(src[n]!=0){
            return src[n];
        }
        int sum = 0 ; 
        int i = 0 ;
        int j = n-1;
        while(j>=i){
            if(j==i){
                sum+=count(j)*count(i);
            }else{
                sum+=2*count(j)*count(i);
            }
            i++;
            j--;
        }
        src[n] = sum;
        return sum;
    }
}

基本思路就是遞歸+狀態記錄