/*
 * @lc app=leetcode.cn id=105 lang=javascript
 *
 * [105] 从前序与中序遍历序列构造二叉树
 */

// @lc code=start
/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {number[]} preorder
 * @param {number[]} inorder
 * @return {TreeNode}
 */
const buildTree = (preorder, inorder) => {
  const helper = (p_start, p_end, i_start, i_end) => {
    if (p_start > p_end) return null;
    let rootVal = preorder[p_start]; // 根节点的值
    let root = new TreeNode(rootVal); // 根节点
    let mid = inorder.indexOf(rootVal); // 根节点在inorder的位置
    let leftNum = mid - i_start; // 左子树的节点数
    root.left = helper(p_start + 1, p_start + leftNum, i_start, mid - 1);
    root.right = helper(p_start + leftNum + 1, p_end, mid + 1, i_end);
    return root;
  };
  return helper(0, preorder.length - 1, 0, inorder.length - 1);
};

// @lc code=end