Tiger's leetcode solution: Remove Duplicate Letters

Tiger's leetcode solution: Remove Duplicate Letters

Q: Given a string which contains only lowercase letters, remove duplicate letters so that every letter appear once and only once. You must make sure your result is the smallest in lexicographical order among all possible results. Example: Given  "bcabc" Return  "abc" Given  "cbacdcbc" Return  "acdb" A: public class Solution { public String removeDuplicateLetters(String s) { Set foundSet = new HashSet(); for(char ch : s.toCharArray()) foundSet.add(ch); return helper(foundSet,new StringBuffer(s)); } private String helper(Set foundSet, StringBuffer buf){ if(buf.length()==0) return ""; Set set = new HashSet(); int i = buf.length()-1 ; for( ;i>=0; i--){ set.add(buf.charAt(i)); if(set.size()== foundSet.size()) break; } // find the min char from 0 to i; inclusive char ch = buf.charAt(0); int index = 0; for(int j = 1 ; j<= i ; j++) if(buf.charAt(j)

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