重建二叉树 九度 Online Judge

#include<iostream>
using namespace std;

class TreeNode
{
public:
char value;
TreeNode *left;
TreeNode *right;
TreeNode(char c): value(c){
left = NULL;
right = NULL;
}
~TreeNode() {
if(left != NULL) delete left;
if(right != NULL) delete right;
}
};

int flag;

TreeNode* buildTree(char *pre, char *mid, int n)
{
int i;
if (n==0) return NULL;
char c = pre[0];
TreeNode *node = new TreeNode(c); //This is the root node of this tree/sub tree.
for(i=0; i<n && mid[i]!=c; i++);
if(mid[i]!=c) flag=1;
int lenL = i; // the node number of the left child tree.
int lenR = n - i -1; // the node number of the rigth child tree.
if(lenL > 0) node->left = buildTree(&pre[1], &mid[0], lenL);
if(lenR > 0) node->right = buildTree(&pre[lenL+1], &mid[lenL+1], lenR);
return node;
}

void PostOrder(TreeNode *p)
{
if(p)
{
PostOrder(p->left);
PostOrder(p->right);
cout<<p->value<<' ';
}
}

int main()
{
int n;
char pre[32],mid[32];
while(cin>>n)
{
flag=0;
for(int i=0;i<n;i++)
{
cin>>pre[i];
}
for(int i=0;i<n;i++)
{
cin>>mid[i];
}
TreeNode *t=buildTree(pre,mid,n);
if(flag==1) cout<<"No";
else PostOrder(t);
cout<<endl;
}
}