剑指 Offer 50. 第一个只出现一次的字符

class Solution {
    
    public char firstUniqChar(String s) {
    
        HashMap<Character,Boolean> dict = new HashMap<>();
        char[] ch = s.toCharArray();
        for(char c:ch){
    
            dict.put(c,!dict.containsKey(c));
        }
        for(char c:ch){
    
            if(dict.get(c)){
    return c;}
        }
        return ' ';
    }
}