leetcode第105题 从前序与中序遍历序列构造二叉树

题目： 给定两个整数数组 preorder 和 inorder ，其中 preorder 是二叉树的先序遍历， inorder 是同一棵树的中序遍历，请构造二叉树并返回其根节点。 题目链接

/**
 * 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}
 */
var buildTree = function(preorder, inorder) {
  return build(preorder, 0, preorder.length - 1, inorder, 0, inorder.length -1);
};

const build = (preorder, preStart, preEnd, inorder, inStart, inEnd) => {
  if (preStart > preEnd) return null;
  const rootValue = preorder[preStart];
  const rootIndex = inorder.indexOf(rootValue);
  const leftTreeSize = rootIndex - inStart;
  const root = new TreeNode(rootValue);
  root.left = build(preorder, preStart + 1, preStart + leftTreeSize, inorder, inStart, rootIndex - 1);
  root.right = build(preorder, preStart + leftTreeSize + 1, preEnd, inorder, rootIndex + 1, inEnd);
  return root;
};

解析；

• 时间复杂度： $O(n)$