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输入: preorder = [3,9,20,15,7], inorder = [9,3,15,20,7]
输出: [3,9,20,null,null,15,7]

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输入: preorder = [-1], inorder = [-1]
输出: [-1]

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// @lc code=start
// Definition for a binary tree node.
// #[derive(Debug, PartialEq, Eq)]
// pub struct TreeNode {
//   pub val: i32,
//   pub left: Option<Rc<RefCell<TreeNode>>>,
//   pub right: Option<Rc<RefCell<TreeNode>>>,
// }
//
// impl TreeNode {
//   #[inline]
//   pub fn new(val: i32) -> Self {
//     TreeNode {
//       val,
//       left: None,
//       right: None
//     }
//   }
// }
use std::cell::RefCell;
use std::rc::Rc;
impl Solution {
    /// ## 解题思路
    /// - 递归
    pub fn build_tree(preorder: Vec<i32>, inorder: Vec<i32>) -> Option<Rc<RefCell<TreeNode>>> {
        fn build_tree(preorder: &[i32], inorder: &[i32]) -> Option<Rc<RefCell<TreeNode>>> {
            inorder
                .iter()
                .enumerate()
                .find(|&v| *v.1 == preorder[0])
                .map(|(i, v)| {
                    Rc::new(RefCell::new(TreeNode {
                        val: *v,
                        left: build_tree(&preorder[1..(i + 1)], &inorder[..i]),
                        right: build_tree(&preorder[(i + 1)..], &inorder[(i + 1)..]),
                    }))
                })
        }
if let Some((i, v)) = inorder.iter().enumerate().find(|&v| *v.1 == preorder[0]) {
                Some(Rc::new(RefCell::new(TreeNode {
                    val: *v,
                    left: build_tree(&preorder[1..(i + 1)], &inorder[..i]),
                    right: build_tree(&preorder[(i + 1)..], &inorder[(i + 1)..]),
                })))
            } else {
                None
            }
        }

        build_tree(&preorder, &inorder)
    }
}
// @lc code=end