求一个字符串的最长重复子序列

string longestRepeatSubstring(const string&str)
{
int n = str.size();
if (n==0)
return NULL;
int maxLength = 0;
int startIndex = 0;

for (int i = 1; i < n; ++i)
{
int current = 0;
for (int j = 0; j < n-i; ++j)
{
if (str[j] == str[i+j])current++;
else
current = 0;
if (current > maxLength)
{
maxLength = current;
startIndex = j-current+1;
}
}
}
if (maxLength > 0)
{
cout << maxI << endl;
return str.substr(startIndex,maxLength);
}
}