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[WIP] 316. Remove Duplicate Letters

Question​

Given a string s, remove duplicate letters so that every letter appears once and only once. You must make sure your result is the smallest in lexicographical order among all possible results.

Example 1:

Input: s = "bcabc"
Output: "abc"

Example 2:

Input: s = "cbacdcbc"
Output: "acdb"

Constraints:

  • 1 <= s.length <= 104
  • s consists of lowercase English letters.

Approach​

Solution​

class Solution {
public:
    string removeDuplicateLetters(string s) {
        vector<int> lastIndex(26, 0);
        for (int i = 0; i < s.length(); i++){
            lastIndex[s[i] - 'a'] = i; // track the lastIndex of character presence
        }
        
        vector<bool> seen(26, false); // keep track seen
        stack<char> st;
        
        for (int i = 0; i < s.size(); i++) {
            int curr = s[i] - 'a';
            if (seen[curr]) continue; // if seen continue as we need to pick one char only
            while(st.size() > 0 && st.top() > s[i] && i < lastIndex[st.top() - 'a']){
                seen[st.top() - 'a'] = false; // pop out and mark unseen
                st.pop();
            }
            st.push(s[i]); // add into stack
            seen[curr] = true; // mark seen
        }
        
        string ans = "";
        while (st.size() > 0){
            ans += st.top();
            st.pop();
        }
        reverse(ans.begin(), ans.end());
        return ans;
    }
};