【LeetCode OJ】Unique Binary Search Trees

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

public class Solution {
public int numTrees(int n) {
if(n == 0 || n == 1)return 1;
int[] f = new int[n+1];
f[0] = f[1] = 1;
for(int i = 2; i < n + 1; ++i){
for(int j = 1; j <= i; ++j){
f[i] += f[i - j] * f[j - 1];
}
}
return f
;
}
}