//利用先序遍历和中序遍历构建二叉树TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {    TreeNode *root=NULL;    if(preorder.size()==0||inorder.size()==0||preorder.size()!=inorder.size())        return root;    root=tree(preorder,0,preorder.size()-1,inorder,0,inorder.size()-1);     return root;} TreeNode* tree(vector<int>&preorder,int preBegin,int preEnd,vector<int>&inorder,int inBegin,int inEnd){    TreeNode *root=NULL;    if(preorder.size()==0||inorder.size()==0||preorder.size()!=inorder.size())        return root;    TreeNode *leftNode=NULL;    TreeNode *rightNode=NULL;    root->val=preorder[preBegin];    int rootVal=root->val;    int i;for(i=inBegin;i<=inEnd;i++)    if(inorder[i]==root->val)            break;    if(i>inEnd)        return root;    int leftLen=i-inBegin;    int rightLen=inEnd-i-1;    leftNode=buildTree(preorder,preBegin+1,preBegin+leftLen,inorder,inBegin,inBegin+leftLen);    rightNode=buildTree(preorder,preBegin+1+leftLen,preEnd,inorder,i+1,i+rightLen);    root->left=leftNode;    root->right=rightNode;    return root;}