通过前序序列与中序序列确定一颗二叉树

能帮忙检查下是哪里出现了问题吗，是内存分配方面的问题吗？

 # include <stdio.h>
# include <string.h>
# include <stdlib.h>

typedef struct tree
{
    char data;
    struct tree * lchild;
    struct tree * rchild;
}Tnode;

void MK(char in[],int ist,int ism,char pre[],int pret,int prem,Tnode** tr)
{
    int i;
    if (ist>ism || pret>prem)
    *tr = NULL;
    else 
    {
        *tr = (Tnode *)malloc(sizeof(Tnode));
        (*tr)->data = pre[pret];
        for (i=ist;i<ism;i++)
        {
            if (in[i] = pre[pret])
            {
                MK(in,ist,i-1,pre,pret+1,pret+i-ist,&(*tr)->lchild);
                MK(in,i+1,ism,pre,pret+i-ist+1,prem,&(*tr)->rchild);
                break;
            }
        }
        if (i>ism)
        printf("输入不合法");
    }

}

void trave (Tnode* tr)
{
    if (tr!=NULL)
    {
        trave(tr->lchild);
        trave(tr->rchild);
        printf ("%c",tr->data);
    }
}

int main ()
{
    char in[20];
    char pre[20];
    Tnode* tr = NULL;

    printf("前序序列");
    gets(in);
    printf("中序序列");
    gets(pre);
    int ism = strlen(in)-1;
    int prem = strlen(pre)-1;

    MK(in,0,ism,pre,0,prem,&tr);
    trave(tr);

    return 0;
}

http://blog.csdn.net/luoluozlb/article/details/52155639