百度面试题：找出给定字符串对应的序号

序列Seq=[a,b,…z,aa,ab…az,ba,bb,…bz,…,za,zb,…zz,aaa,…] 类似与excel的排列，任意给出一个字符串s=[a-z]+(由a-z字符组成的任意长度字符串），请问s是序列Seq的第几个。

注意到每满26个就会向前进一位，类似一个26进制的问题。
比如ab，则位置为26*1 + 2；
比如za，则位置为26*26 + 1；
比如abc，则位置为26*26*1 + 26*2 + 3；

#include<iostream>
#include <cstdlib>

using namespace std;

unsigned int position(char *str, int len)
{
assert(str != NULL && len > 0);
char c = *str;
if(len == 1) return c - 96;

int i = 0;
int col_position = str[len - 1] - 96; // 最后一个字符的大小

unsigned int zzz_position = 1;
for(i = 0; i < len - 1; i++)
{
zzz_position *= 26;
}

unsigned int row_position = position(str, len - 1); // 递归

return zzz_position + (row_position - 1)*26 + col_position;
}

/*
* a b c d ... z
* aa ab ac ad ... az
* ba bb bc bd ... bz
* ...
* za zb zc zd ... zz
* ...
* aaa aab aac ... aaz
* aba abb abc ... abz
* ...
* zza zzb zzc ... zzz
* aaaa ...
*/

int main()
{
cout << position("abcd", 4) << endl;
system("pause");
return 0;
}

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