算法 |《剑指offer》面试题 07. 重建二叉树

输入某二叉树的前序遍历和中序遍历的结果，请重建该二叉树。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，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

限制：

0 <= 节点个数 <= 5000

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) {
        int n = preorder.length;
        if(n == 0)  return null;
        int rootVal = preorder[0], rootIndex = 0;
        for(int i = 0 ;i < n; i++) {
            if(inorder[i] == rootVal) {
                rootIndex = i;
                break;
            }
        }
        TreeNode root = new TreeNode(rootVal);
        root.left = buildTree(Arrays.copyOfRange(preorder, 1, 1 + rootIndex), Arrays.copyOfRange(inorder, 0, rootIndex));
        root.right = buildTree(Arrays.copyOfRange(preorder, rootIndex + 1, n), Arrays.copyOfRange(inorder, rootIndex + 1, n));
        return root;
    }
}