真二叉树重构(Proper Rebuild)

#include 
const int SZ = 1 << 20;  //提升IO buff   
struct fastio{  
    char inbuf[SZ];  
    char outbuf[SZ];  
    fastio(){  
        setvbuf(stdin, inbuf, _IOFBF, SZ);  
        setvbuf(stdout, outbuf, _IOFBF, SZ);  
    }  
}io; 
const int maxn = 4000010;
int pre[maxn], post[maxn], in[maxn];
int n;
struct node
{
    node *lchild, *rchild;
    int num;
};
node* createtree (int pre_lo, int pre_hi, int post_lo, int post_hi)
{
    node* root = new node;
    root->num = pre[pre_lo];
    root->rchild = NULL;
    root->lchild = NULL;
    if(post_lo == post_hi)
        return root;
    int pos;
    for(int i = post_lo; i <= post_hi; i++)
    {
        if(post[i] == pre[pre_lo + 1])
        {
            pos = i;
            break;
        }
    }
    int numleft = pos - post_lo + 1;
    root->lchild = createtree(pre_lo + 1, pre_lo + numleft, post_lo, pos);
    root->rchild = createtree(pre_lo + numleft + 1, pre_hi, pos + 1, post_hi - 1);
    return root;
}
void inOrder(node *root)
{
    if(root->lchild != NULL)
        inOrder(root->lchild);
    printf("%d ", root->num);
    if(root->rchild != NULL)
        inOrder(root->rchild);
}
int main()
{
    scanf("%d", &n);
    for(int i = 0; i < n; i++)
    {
        scanf("%d", &pre[i]);
    }
    for(int i = 0; i < n; i++)
    {
        scanf("%d", &post[i]);
    }
    node* root = createtree(0, n-1, 0, n-1);
    inOrder(root);
    return 0;
}