C++ 代码

/**
 * 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* buildTree(vector<int>& preorder, vector<int>& inorder) {
        int n = preorder.size();
        for(int i = 0; i < n; ++ i) hash[inorder[i]] = i; 
        return dfs(0, 0, n - 1, preorder);
    }

    TreeNode* dfs(int cur, int start, int end, vector<int>& pre){
        if(start > end) return nullptr;
        TreeNode* root = new TreeNode(pre[cur]);
        if(start == end) return root;

        int mid = hash[pre[cur]]; // 找到当前根节点在中序序列中的位置
        int len = mid - start; // 左子树的长度

        root->left = dfs(cur + 1, start, mid - 1, pre);
        root->right = dfs(cur + len + 1, mid + 1, end, pre);

        return root;
    }
};