Arrange an Array to Form a Smallest Digit

/**
 * Input an array of positive integers, arrange the integers to form new digits,
 * and output the smallest digit among all the new ones.
 * Input Example 1:
 * {2, 1}
 * Output Example 1:
 * 12
 *
 * Input Example 2:
 * {32, 321}
 * Output Example 2:
 * 32132
 *
 * Input Example 3:
 * {4589, 101,41425,9999}
 * Output Example 3:
 * 1014142545899999;
 */

// 功能：将输入的数组排成最小的数
// 输入： int a[]:整型数组
//       int nCount:数组长度
//       char * strRst 返回值
// 输出：
// 返回：成功返回0  异常返回-1

#include <iostream>
#include <string>
#include <vector>
#include <sstream>
#include <algorithm>

using namespace std;

bool cmp(string str1, string str2)
{
    return (str1 + str2 < str2 + str1);
}

int smallestDigit(int a[], int nCount, char *strRst)
{
    if(NULL == a || nCount <= 0 || NULL == strRst)
    {
        return -1;
    }

    vector<string> numStrs;
    stringstream strStream;
    string num;
    /*! int转string */
    for(int i = 0; i < nCount; ++i)
    {
        strStream << a[i];
        strStream >> num;
        strStream.clear();
        numStrs.push_back(num);
    }

    sort(numStrs.begin(), numStrs.end(), cmp);

    int len  = 0;
    for(int i = 0; i < numStrs.size(); ++i)
    {
        len = numStrs[i].size();
        strncpy(strRst, numStrs[i].c_str(), len);
        strRst += len;
    }
    *strRst = '';

    return 0;
}

int main()
{
    int a[] = {4589, 101, 41425, 9999};
    char buf[1000];
    smallestDigit(a, 4, buf);
    cout << buf << endl;

    return 0;
}