第一个只出现一次的字符

class Solution {
public:
    int FirstNotRepeatingChar(string str) {
        char ch;
        int j;
        for(int i = 0;i<str.length();i++){
            ch = str[i];
            for(j=0;j<i;j++){
                if(ch == str[j])
                    break;
            }
            if(j == i)
                for(j=i+1;j<str.length();j++){
                    if(ch == str[j])
                        break;
                }
            if(j == str.length())
                return i;
        }
        return -1;
    }
};

class Solution {
    public char firstUniqChar(String s) {
        if(s.length()==0) return ‘ ‘;
        HashMap<Character,Integer> mp = new LinkedHashMap<>();
        int value;
        
        for(int i = 0; i < s.length(); i++){
            if(mp.get(s.charAt(i))==null)
                mp.put(s.charAt(i),1);
            else{
                value = mp.get(s.charAt(i));
                mp.put(s.charAt(i),++value);
            }
        }
        for(Character ch:mp.keySet()){
            value = mp.get(ch);
            if(value == 1)
                return ch;
        }
        return ‘ ‘;
    }
}

class Solution {
public:
    char firstUniqChar(string s) {
        unordered_map<char,int> mp;
        for(int i=0;i<s.length();i++){
            mp.insert(make_pair(s[i],0));
        }
        for(int i=0;i<s.length();i++){
            mp[s[i]]++;
        }
        for(int i=0;i<s.length();i++){
            if(mp[s[i]] == 1)
                return s[i];
        }
        return ‘ ‘;
    }
};