316. Remove Duplicate Letters 🌟🌟

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.

NOTE: This question is same as 1081

Solution

• We store the frequency of characters in a hashmap or vector.
• Create an answer string to store the result.
• for every character in the string.
  • Decrement the frequency of the character.
  • If its already visited the continue.
  • else remove all characters from ans string that are greater than current character.
  • Add current character in the answer string.
  • mark visited as true.
• Return the answer string.
• TC: O(N)
• SC: O(26)=O(1)

Code

class Solution {
public:
    string removeDuplicateLetters(string s)
    {
        int n = s.size();
        vector<int> cnt(26, 0); // frequencies
        vector<bool> vis(26, false); // visited

        // count the frequencies of characters form string
        for (char c : s)
            cnt[c - 'a']++;

        string ans;
        for (char c : s) {
            cnt[c - 'a']--; // decrease the frequency of current character
            if (vis[c - 'a'] == true)
                continue;
            // remove all characters in ans that are greater than c
            while (!ans.empty() && ans.back() > c && cnt[ans.back() - 'a'] >= 1) {
                vis[ans.back() - 'a'] = false;
                ans.pop_back();
            }
            // add c in the ans string
            ans += c;
            vis[c - 'a'] = true;
        }
        return ans;
    }
};```