105. 从前序与中序遍历序列构造二叉树-中等难度

问题描述

根据一棵树的前序遍历与中序遍历构造二叉树。

注意:
你可以假设树中没有重复的元素。

例如，给出

前序遍历 preorder = [3,9,20,15,7]
中序遍历 inorder = [9,3,15,20,7]
返回如下的二叉树：

3
/
9 20
/
15 7

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal

解答

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int find(int[] a,int key){
        boolean flag = false;
        int index = 0;
        for(index=0;index<a.length;index++){
            if(a[index]==key){
                flag = true;
                break;
            }
        }
        if(flag)return index;
        return -1;
    }
    public TreeNode recursive(int[] pre, int[] in){
        if(in.length==0)return null;
        if(in.length==1)return new TreeNode(in[0],null,null);
        TreeNode r = null;
        int position = find(in,pre[0]);
        TreeNode l = recursive(Arrays.copyOfRange(pre, 1, position+1),Arrays.copyOfRange(in, 0, position));
        if(position+1<in.length)
        r = recursive(Arrays.copyOfRange(pre, position+1, pre.length),Arrays.copyOfRange(in, position+1, in.length));
        return new TreeNode(pre[0],l,r);
    }
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        if(preorder.length==0)return null;
        return recursive(preorder,inorder);
    }
}