一些分界点自己手动模拟一下会比较清楚

/**
 * 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:
    vector<int> preorder, inorder;
    map<int, int> mp;
    TreeNode* buildTree(vector<int>& _preorder, vector<int>& _inorder) {
        preorder = _preorder, inorder = _inorder;
        for(int i = 0; i < inorder.size(); i ++ )   mp[inorder[i]] = i;
        return dfs(0, preorder.size() - 1, 0, inorder.size() - 1);
    }

    TreeNode* dfs(int pl, int pr, int il, int ir){
        if(pl > pr) return NULL;
        TreeNode* root = new TreeNode(preorder[pl]);
        int p = mp[preorder[pl]];
        root->left = dfs(pl + 1, pl + 1 + p - il - 1, il, p - 1);
        root->right = dfs(pl + 1 + p - il - 1 + 1, pr, p + 1, ir);
        return root;
    }
};