[LeetCode] First Unique Character in a String

1. 문제 : https://leetcode.com/problems/first-unique-character-in-a-string/description/

 

2. 풀이

알파벳 나오면 무조건 alpha - 'a' 해서 숫자로 만드는거 생각하기

 

3. 코드

class Solution {
    public int firstUniqChar(String s) {
        int[] alph = new int[26];
        //char -> first index pairs
        Map<Character, Integer> map = new HashMap<>();
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            //count char occurences
            alph[c - 'a']++;
            //this if statement is used for optimization
            //if keySet().size() = 26, then we no longer 
            //call map's methods
            if (map.keySet().size() < 26) {
                if (!map.containsKey(c)) {
                    map.put(c, i);
                }
            }
        }
        int minIdx = s.length();
        //finding minimum index of char, whose occurence is 1.
        for (char c : map.keySet()) {
            if (alph[c - 'a'] == 1) {
                int idx = map.get(c);
                minIdx = Math.min(minIdx, idx);
            }
        }
        return minIdx == s.length() ? -1 : minIdx;
    }
}