按ASCII码给定字符串序号

,我已经建好了哈夫曼树可以根据输入叶子结点个数和权值得出正确哈夫曼编码结果，但怎样把输入的字符串根据ASCII给定它的序号

仅供参考：

#include <iostream>
#include <string>
using namespace std;
struct huffTree {
    int parent;
    int lchild;
    int rchild;
    int weight;
    string flag;
};
struct Lowest_node {
    char ch;
    int ch_num;
};
void coding(int length,huffTree *tree,int n,int &a,int &b) {
    int i;
    int r,s;

    r=s=length;
    for (i=0;i<n;i++) {
        if (tree[i].weight<r
         && tree[i].parent==-1) {
            r=tree[i].weight;
            a=i;
        }
    }
    for (i=0;i<n;i++) {
        if (tree[i].weight<s
         && i!=a
         && tree[i].parent==-1) {
            s=tree[i].weight;
            b=i;
        }
    }
}
void frequency(string str) {
    int i,j;
    int length=str.length();
    Lowest_node *node=new Lowest_node[length];

    for (i=0;i<length;i++) node[i].ch_num=0;

    int char_type_num=0;
    for (i=0;i<length;i++) {
        for (j=0;j<char_type_num;j++)
            if (str[i]==node[j].ch
            || ('a'<=node[j].ch && node[j].ch<='z'
                && str[i]+32==node[j].ch))
                break;//
        if (j<char_type_num) node[j].ch_num++;
        else {
            if ('A'<=str[i] && str[i] <= 'Z') node[j].ch=str[i]+32;
            else node[j].ch=str[i];
            node[j].ch_num++;
            char_type_num++;
        }
    }
    for (i=0;i<char_type_num;i++) {
        for (j=i;j<char_type_num;j++) {
            if (node[j].ch_num<node[j+1].ch_num) {
                int temp;
                char ch_temp;
                temp=node[j].ch_num;
                ch_temp=node[j].ch;
                node[j].ch_num=node[j+1].ch_num;
                node[j].ch=node[j+1].ch;
                node[j+1].ch_num=temp;
                node[j+1].ch=ch_temp;
            }
        }
    }
    for (i=0;i<char_type_num;i++)
        cout<<"字符"<<node[i].ch<<"出现了"<<node[i].ch_num<<"次"<<endl;
    huffTree *huff=new huffTree[2*char_type_num-1];
    huffTree temp;
    string *code=new string[2*char_type_num-1];

    for (i=0;i<2*char_type_num-1;i++) {
        huff[i].lchild=-1;
        huff[i].parent=-1;
        huff[i].rchild=-1;
        huff[i].flag=-1;
    }
    for (j=0;j<char_type_num;j++) huff[j].weight=node[j].ch_num;
    int min1,min2;
    for (int k=char_type_num;k<2*char_type_num-1;k++) {
        coding(length,huff,k,min1,min2);
        huff[min1].parent=k;
        huff[min2].parent=k;
        huff[min1].flag="0";
        huff[min2].flag="1";
        huff[k].lchild=min1;
        huff[k].rchild=min2;
        huff[k].weight=huff[min1].weight+huff[min2].weight;
    }
    for (i=0;i<char_type_num;i++) {
        temp=huff[i];
        while (1) {
            code[i]=temp.flag+code[i];
            temp=huff[temp.parent];
            if (temp.parent==-1) break;//
        }
    }
    cout<<"字符串的每个字符huffman编码为："<<endl;
    for (i=0;i<char_type_num;i++) cout<<node[i].ch<<"  "<<code[i]<<endl;
    cout<<"整个字符串的huffman编码为："<<endl;
    for (i=0;i<length;i++) {                                                                                     //S?
        for (j=0;j<char_type_num;j++) {
            if (str[i]==node[j].ch)
                cout<<code[j];
        }
    }
    delete[] node;
    node=NULL;
    delete[] huff;
    huff=NULL;
    delete[] code;
    code=NULL;
}
int main() {
    int length=0;
    string str;
    cout<<"请输入一个字符串：";
    cin>>str;
    frequency(str);
    return 0;
}
//请输入一个字符串：2333abcde
//字符3出现了3次
//字符2出现了1次
//字符a出现了1次
//字符b出现了1次
//字符c出现了1次
//字符d出现了1次
//字符e出现了1次
//字符串的每个字符huffman编码为：
//3  11
//2  000
//a  001
//b  010
//c  011
//d  100
//e  101
//整个字符串的huffman编码为：
//000111111001010011100101