Leetcode Remove Duplicate Letters problem solution

class Solution(object):
    def removeDuplicateLetters(self, s):
        """
        :type s: str
        :rtype: str
        """
        if not s:
            return ""
        stack = []
        cnt = collections.Counter(s)
        for ch in s:
            if ch not in stack:
                while stack and ch <= stack[-1] and cnt[stack[-1]] > 1:
                    cnt[stack.pop()] -=1
                stack.append(ch)
            else:
                cnt[ch] -=1
        return "".join(stack)

StringBuilder sb = new StringBuilder();
    int[] count = new int[26];
    
    public String removeDuplicateLetters(String s) { 
         
        char[] schar = s.toCharArray(); 
        for(char c : schar){
            count[c - 'a']++;
        }
        helper(schar, 0); 
        return sb.toString(); 
    }
    
    private void helper(char[] schar, int start){ 
        
        if(start == schar.length) return;  
        int pos = -1, i = start;
        char cpos = 255;
        for(;i < schar.length; i++) {
            if(count[schar[i] - 'a'] == -1) continue; 
            if (schar[i] < cpos){
                pos = i;
                cpos = schar[i];
            }
            if (--count[schar[i] - 'a'] == 0){
                i++;
                break;
            }  
        } 
        
        if(pos == -1) return;
        sb.append(cpos);
        count[cpos - 'a'] = -1;
        for(int n = pos + 1; n < i; n++){ 
            if(count[schar[n] - 'a'] != -1) count[schar[n] - 'a']++;
        } 
        helper(schar, pos + 1); 
    }

string removeDuplicateLetters(string s){
    
    vector<bool> exist(26,false);
    vector<int> freq(26,0);
    stack<char> st;
    string res;
    if(s.length()==1)   return s;
    
    for(auto ch: s){
        freq[ch-'a']++;
    }
    
    for(int i=0; i<s.length(); i++){
        char ch= s[i];
        
        freq[ch-'a']--;
        if(exist[ch-'a']){
            continue;
        }
        
        while(st.size()>0 && st.top()>ch && freq[st.top()-'a']>0 ){
            char top= st.top();
            st.pop();
            exist[top-'a']= false;
        }
        st.push(ch);
        exist[ch-'a']= true;
    }
    while(st.size()>0){
        res.push_back(st.top());
        st.pop();
    }
    reverse(res.begin(),res.end());
    
    return res;
}
};

char* removeDuplicateLetters(char* s)
{
    int len = strlen(s);
    if (len < 2)
        return s;
    int count[26] = { 0 }, choosen[26] = { 0 }, rlen = 0;
    for (int i = 0; i < len; i++) {
        count[s[i] - 'a']++;
        if (count[s[i] - 'a'] == 1)
            rlen++;
    }
    char* result = (char*)malloc(sizeof(char) * (rlen + 1));
    for (int i = 0, j = 0; i < len; i++) {
        count[s[i] - 'a']--;
        if (choosen[s[i] - 'a'])
            continue;
        while (j > 0 && s[i] < result[j - 1] && count[result[j - 1] - 'a'] > 0) {
            choosen[result[j - 1] - 'a'] = 0;
            j--;
        }
        result[j++] = s[i];
        choosen[s[i] - 'a'] = 1;
    }
    result[rlen] = '';
    return result;
}