重建二叉树
以后这就是我的模板了（）
/**
 * 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:
    TreeNode* buildTree(vector<int>&preorder, vector<int>&inorder) {
        TreeNode* root=new TreeNode(0);
        if(preorder.size()==0) return NULL;
        dfs(preorder,inorder,root);
        return root;
    }
    void dfs(vector<int>A,vector<int>B,TreeNode* root){
        if(A.size()==0) return ;
        int num=A[0];
        root->val=A[0];
        vector<int>a_lt,a_rt;
        vector<int>b_lt,b_rt;
        int flag=0;
        for(int i=0;i<A.size();i++)
        {
            if(B[i]==num)
            {
                flag=1;
                continue;
            }
            if(flag) b_rt.push_back(B[i]);
            else b_lt.push_back(B[i]);
        }
        for(int i=1;i<A.size();i++)
        {
            if(i<=b_lt.size()) a_lt.push_back(A[i]);
            else a_rt.push_back(A[i]);
        }
        if(a_lt.size()>0)
        {
            root->left=new TreeNode(0);
            dfs(a_lt,b_lt,root->left);
        }
        if(a_rt.size()>0)
        {
            root->right=new TreeNode(0);
            dfs(a_rt,b_rt,root->right);
        }
    }
};