pub fn catalan(node_num: u64) -> u64 {
    let mut c: f64 = 1.0;
    for i in 0..node_num {
        c = c * 2.0 * (2.0 * i as f64 + 1.0) / (i as f64 + 2.0);
    }
    c as u64
}
// catalan calculate how many binary search trees can be formed by node_num nodes
catalan calculate how many binary search trees can be formed by node_num nodes
catalan calculate how many binary search trees can be formed by node_num nodes
pub fn catalan(node_num: u64) -> u64 {
    let mut c: f64 = 1.0;
    for i in 0..node_num {
        c = c * 2.0 * (2.0 * i as f64 + 1.0) / (i as f64 + 2.0);
    }
    c as u64
}
pub fn catalan ( node_num : u64 ) -> u64 { let mut c : f64 = 1.0 ; for i in 0 .. node_num { c = c * 2.0 * ( 2.0 * i as f64 + 1.0 ) / ( i as f64 + 2.0 ) ; } c as u64 }
catalan calculate how many binary search trees can be formed by node_num nodes
catalan calculate how many binary search trees can be formed by node_num nodes
node_num
u64