Minimum window substring TLE O(n+m)

class Solution {
public:
    int find_ind(char curr_char)
    {
        int ind=0;
        if(isupper(curr_char))
        {
           ind=26+(curr_char-'A');
        }
        else
        {
            ind=curr_char-'a';
        }
        return ind;
    }
    bool comp(vector<int> &vs,vector<int> &vt,char curr_char)
    {
        int ind=find_ind(curr_char);
        if(vs[ind]==vt[ind]) return true;
        else                return false;
    }
    bool less(vector<int> &vs,vector<int> &vt,char curr_char)
    {
        int ind=find_ind(curr_char);
        if(vs[ind]==vt[ind]-1) return true;
        else                return false;
    }
    bool find_vt(vector<int> &vt,char curr_char)
    {
        int ind=find_ind(curr_char);
        if(vt[ind]>0) return 1;
        return 0;
    }
    void change(vector<int> &v,string str,int ind,int val)
    {
        if(isupper(str[ind]))
        {
          // cout<< v[(str[ind]-'A')+26]<<" ";
            v[(str[ind]-'A')+26]+=val;
           // cout<<v[(str[ind]-'A')+26]<<endl;
        }
        else
        {
         // cout<<  v[str[ind]-'a']<<" ";
            v[str[ind]-'a']+=val;
         // cout<<  v[str[ind]-'a']<<endl;
        }
    }

    string minWindow(string s, string t) {
    vector<int> vs(52,0),vt(52,0) ;
    int m=t.size();
    for(int i=0;i<m;i++)
    {
        change(vt,t,i,1);//incrementing by 1
    }
    int required=0;
    for(auto x:vt) if(x) required++;
    int n=s.size();
    int left=-1,right=0;
    string ans="",finalans="";
    bool nextdec=false;
    char curr_char;
    int have=0;
    while(left<right&&right<n)
    {

        if(nextdec)
        {
            change(vs,s,left,-1);//decrementing by 1
            curr_char=s[left];
            if(less(vs,vt,curr_char)) have--;

        }
        else
        {
             change(vs,s,right,1);//incrementing by 1
             curr_char=s[right];
             if(comp(vs,vt,curr_char)) have++;

        }
        nextdec=false;//releasing if locked
        if(have==required)
        {
             {
                finalans=ans;
                ans=s.substr(left+1,right-left);
                int fanssz=finalans.size();
                if(fanssz)
                {
                    if(fanssz<ans.size())//choosing the minimum answer
                        ans=finalans;
                }


             }
            nextdec=true;//locking the next iterartion to be as a decrement one
            left++;
        }
        else
        {
            right++;
        }
       // cout<<ans<<endl;

    }
        return ans;
    }
};