重建二叉树-剑指offer

输入某二叉树的前序遍历和中序遍历的结果，请重建出该二叉树。假设输入的前序遍历和中序遍历的结果中都不含重复的数字。例如输入前序遍历序列{1,2,4,7,3,5,6,8}和中序遍历序列{4,7,2,1,5,3,8,6}，则重建二叉树并返回。

https://www.nowcoder.com/questionTerminal/8a19cbe657394eeaac2f6ea9b0f6fcf6

public class TreeNode {int val;TreeNode left;TreeNode right;TreeNode(int x) {val = x;}}public TreeNode help(int[] pre, int pl, int pr, int[] in, int il, int ir) {if (pl > pr || il > ir) {return null;}TreeNode root = new TreeNode(pre[pl]);int len = 0;for (int i = il; i <= ir; i++) {if (in[i] == pre[pl]) {break;}len++;}root.left = help(pre, pl + 1, pl + len, in, il, il + len - 1);root.right = help(pre, pl + len + 1, pr, in, il + len + 1, ir);return root;}public TreeNode reConstructBinaryTree(int[] pre, int[] in) {TreeNode root = help(pre, 0, pre.length - 1, in, 0, in.length - 1);return root;}