2. < tag hash table, string> supplement: Sword finger offer 50 The first character DBC that appears only once

The finger of the sword Offer 50. The first character that appears only once

[ Case needs ]

[ Train of thought analysis 1 , Two traversal ]

[ Code implementation ]

class Solution {
    
    public char firstUniqChar(String s) {
    
        Map<Character, Integer> frequency = new HashMap<Character, Integer>();
        for (int i = 0; i < s.length(); ++i) {
    
            char ch = s.charAt(i);
            frequency.put(ch, frequency.getOrDefault(ch, 0) + 1);
        }
        for (int i = 0; i < s.length(); ++i) {
    
            if (frequency.get(s.charAt(i)) == 1) {
    
                return s.charAt(i);
            }
        }
        return ' ';
    }
}

[ Train of thought analysis II , utilize API]

class Solution {
    
    public char firstUniqChar(String s) {
    
        for(int i = 0; i< s.length(); i++){
    
            char value = s.charAt(i);
            if(s.indexOf(value)==s.lastIndexOf(value)){
    
                return value;
            }
        }
        return ' '; 
    }
}