105 - 从前序与中序遍历序列构造二叉树

题目描述

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：

链接：https://leetcode-cn.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal

题解

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        if(preorder == null || preorder.length == 0) {
            return null;
        }
        TreeNode node = new TreeNode(preorder[0]);
        int mid = 0;
        for(int i = 0; i < preorder.length; i++) {
            if(preorder[0] == inorder[i]){
                mid = i;
                break;
            }
        }
        int[] leftPreorder = Arrays.copyOfRange(preorder, 1, mid + 1);
        int[] rightPreorder = Arrays.copyOfRange(preorder, mid + 1, preorder.length);
        int[] leftInorder = Arrays.copyOfRange(inorder, 0, mid);
        int[] rightInorder = Arrays.copyOfRange(inorder, mid + 1, inorder.length);
        node.left = buildTree(leftPreorder, leftInorder);
        node.right = buildTree(rightPreorder, rightInorder);
        return node;
    }
}