知道二叉树前序和中序序列打印后序序列

题目：知道一棵二叉树的前序序列、中序系列，打印出它的后序系列
下面算法的时间复杂度：O(n^2) -- 最坏情况就是左单支树

/*
    Kown the preorder listing and the inorder listing of a tree, print it's postorder listing
*/
void PrintPostOrder(const char *pre, int s1, int e1,
                    const char *in,  int s2, int e2)
{
    if (s1 > e1 || s2 > e2)
        return;
    char root = pre[s1];
    for (int i=s2; i<=e2; i++)
        if (in[i] == root)
            break;
    int offset = i-s2;
    PrintPostOrder(pre, s1+1, s1+offset, in, s2, i-1);
    PrintPostOrder(pre, s1+offset+1, e1, in, i+1, e2);
    printf("%c", root);
}
int main()
{
    const char *pre="ABCDEF";
    const char *in="FEDCBA";
    printf("PreOrder : %s\n", pre);
    printf("InOrder  : %s\n", in);
    printf("PostOrder: ");
    PrintPostOrder(pre, 0, strlen(pre)-1, in, 0, strlen(in)-1);
    printf("\n");

    return 0;
}