给定两个整数数组 preorder 和 inorder ，其中 preorder 是二叉树的先序遍历， inorder 是同一棵树的中序遍历，请构造二叉树并返回其根节点。

示例 1:

输入: preorder = [3,9,20,15,7], inorder = [9,3,15,20,7]
输出: [3,9,20,null,null,15,7]

示例 2:

输入: preorder = [-1], inorder = [-1]
输出: [-1]

代码：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
struct TreeNode* buildTree(int* preorder, int preorderSize, int* inorder, int inorderSize) {
    if(preorderSize<=0||inorderSize<=0) return NULL;
    struct TreeNode *root=(struct TreeNode*)malloc(sizeof(struct TreeNode));
    int index;
    root->val=preorder[0];
    for(index=0;index<inorderSize;index++){
        if(inorder[index]==preorder[0]){
            break;
        }
    }
    root->left=buildTree(preorder+1,index,inorder,index);
    root->right=buildTree(preorder+1+index,preorderSize-1-index,inorder+1+index,inorderSize - index - 1);
    return root;
}