105.从前序与中序遍历序列构造二叉树.js

/*
 * @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}
 */
var buildTree = function(preorder, inorder) {
  const build = (preorder, preStart, preEnd, inorder, inStart, inEnd) => {
    if (preStart > preEnd) return null;

    // root 节点对应的值就是前序遍历数组的第一个元素
    const rootVal = preorder[preStart];
    // rootVal 在中序遍历数组中的索引
    let index = 0;

    // 注意这里是 从 inStart 到 inEnd
    for (let i = inStart; i <= inEnd; i++) {
      if (inorder[i] === rootVal) {
        index = i;
        break;
      }
    }

    const root = new TreeNode(rootVal);
    const leftSize = index - inStart;

    root.left = build(preorder, preStart + 1, preStart + leftSize, inorder, inStart, index - 1);
    root.right = build(preorder, preStart + leftSize + 1,  preEnd, inorder, index + 1, inEnd)

    return root;
  }
  // 这里记得是 length - 1
  return build(preorder, 0, preorder.length - 1, inorder, 0, inorder.length - 1);
};
// @lc code=end