Permutation in String

class Solution {
public:
    bool ispermutation(string str1, string str2)
{
    // Create 2 count arrays and initialize
    // all values as 0
    int count1[256] = {0};
    int count2[256] = {0};
    int i;
 
    // For each character in input strings,
    // increment count in the corresponding
    // count array
    for (i = 0; str1[i] && str2[i];  i++)
    {
        count1[str1[i]]++;
        count2[str2[i]]++;
    }
 
    // If both strings are of different length.
    // Removing this condition will make the
    // program fail for strings like "aaca"
    // and "aca"
    if (str1[i] || str2[i])
      return false;
 
    // Compare count arrays
    for (i = 0; i < 256; i++)
        if (count1[i] != count2[i])
            return false;
 
    return true;
}
    bool checkInclusion(string s1, string s2) {
        int n=s1.size(),m=s2.size();
        for(int i=0;i<m-n+1;i++){
            if(ispermutation(s2.substr(i,n),s1)){
                return true;
            }
        }
        return false;
    }
};