重建二叉树

输入一棵二叉树前序遍历和中序遍历的结果，请重建该二叉树。

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    unordered_map<int, int> hash;
        
    TreeNode* build (vector<int>& pre, vector<int>& in, int pl, int pr, int il, int ir) {
        if (pl > pr) return NULL;
        
        auto root = new TreeNode(pre[pl]);
        int k = hash[root->val];
        
        if (il < k) root->left = build (pre, in, pl + 1, k - il + pl, il, k - 1);
        if (k < ir) root->right = build (pre, in, k - il + pl + 1, pr, k + 1, ir);
        
        return root;
    }
    
    TreeNode* buildTree(vector<int>& pre, vector<int>& in) {
        for (int i = 0; i < in.size(); i++) {
            hash[in[i]] = i;
        }
        
        auto root = build (pre, in, 0, pre.size() - 1, 0, in.size() - 1);
        return root;
    }
};