1 public class Solution { 2 public String removeDuplicateLetters(String s) { 3 if (s.length() < 2) { 4 return s; 5 } 6 int[] letters = new int[26]; 7 for (char c : s.toCharArray()) { 8 letters[c - 'a']++; 9 }10 boolean[] visited = new boolean[26];11 StringBuilder result = new StringBuilder(" ");12 for (char c : s.toCharArray()) {13 letters[c - 'a']--;14 if (visited[c - 'a']) {15 continue;16 }17 visited[c - 'a'] = true;18 while (c < result.charAt(result.length() - 1) && letters[result.charAt(result.length() - 1) - 'a'] > 0) {19 visited[result.charAt(result.length() - 1) - 'a'] = false;20 result.setLength(result.length() - 1);21 }22 result.append(c);23 }24 return result.toString().substring(1);25 }26 } 1. Add " " to ensure the comparision works. Or add a condition that result.length() > 0
2. character counting decrease happens before check it has been visited or not.
