【二叉树】根据一棵树的前序遍历与中序遍历构造二叉树

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]

 

class Solution {
    
    public static TreeNode restore(int[] pre, int[] mid){
        if(null == pre || null == mid){
            return null;
        }

        return handle(pre, 0, pre.length; mid, 0, mid.length);
    }

    private static TreeNode handle(int[] pre,int begin1, int end1, int[] in, int begin2, int end2){
        if(begin1 > end1 || end1 >= pre.length || begin1 < 0){
            return null;
        }

        int val = pre[begin1];
        int mid = begin2;
        for(int i = begin2; i < end2; i++){
            if(val == in[i]){
                mid = i;
                break;
            }
        }

        TreeNode result = new TreeNode(val);
        result.left = handle(pre, begin1 + 1, begin1 + (mid - begin2), in, begin2, mid - 1);
        result.right = handle(pre, end1 - (end2 - mid) + 1, end1, in, mid + 1, end2);
        return result;
    }
}