316. Remove Duplicate Letters

Problem Statement

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.

Note: This question is the same as 1081: https://leetcode.com/problems/smallest-subsequence-of-distinct-characters/

Intuition

Approach:
We want increasing (lexographic distinct) String, 
Thats where we think of the stack, Monotonic

Now, We make a note of all characters, Last appearing index in the map
Then we push the elements in stack,
When, 

smaller element comes in stack , top > current
We have to pop, but we only pop only if that popping element is present at higher
index, otherwise not

Links

https://leetcode.com/problems/remove-duplicate-letters/description/

Video Links

Approach 1:

C++

class Solution {
public:
    string removeDuplicateLetters(string s) {
        stack<char> st;
        unordered_map<char,int> mp;
        vector<bool> track(26,false);
        for(int i=0; i<s.size(); i++)
            mp[s[i]] = i;

        string ans="";
        for(int i=0; i<s.size(); i++){
            if(track[s[i]-'a'] == true)
                continue;
                
            while(!st.empty() and st.top() > s[i] and i<mp[st.top()]){
                track[st.top()-'a'] = false;
                st.pop();
            }

            st.push(s[i]);
            track[s[i]-'a'] = true;
        }

        while(!st.empty()){
            ans += st.top();
            st.pop();
        }

        reverse(ans.begin(), ans.end());
        return ans;
    }
};

Approach 2:

C++

Approach 3:

C++

Approach 4:

C++