Leecode 1008. 前序遍历构造二叉搜索树 递归

原题链接：Leecode 1008. 前序遍历构造二叉搜索树

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    TreeNode* bstFromPreorder(vector<int>& preorder) {
        if(preorder.size()==0) return nullptr;
        TreeNode* root=new TreeNode(preorder[0]);
        if(preorder.size()==1) return root;
        int k=0;
        for(int i=1;i<preorder.size();i++)
        {
            if(preorder[i]<preorder[0]) k=i;
            else break;
        }
        vector<int> lpreorder,rpreorder;
        lpreorder.assign(preorder.begin()+1,preorder.begin()+k+1);
        rpreorder.assign(preorder.begin()+k+1,preorder.end());
        root->left=bstFromPreorder(lpreorder);
        root->right=bstFromPreorder(rpreorder);
        return root;
    }
};