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#ifdef LOCAL
#include <bits/stdc++.h>
using namespace std;
// tree node stuff here...
#endif
static int __initialSetup = []()
{
std::ios::sync_with_stdio(false);
cin.tie(NULL);
return 0;
}
();
class Solution
{
public:
string removeDuplicateLetters(string s)
{
int cnt[26] = {0};
for (auto i : s)
cnt[i - 'a']++;
bool in[26] = {false};
stack<char> st;
for (auto cc : s) {
int c = cc - 'a';
cnt[c]--; // oops
if (in[c]) // no need to insert again
continue;
// observation
// if the current character is better than the top character in the stack
// and
// the top character still has occurrence after the current one:
// pop it
while (st.size() > 0 && cc < st.top() && cnt[st.top() - 'a'] > 0) {
// cnt[st.top() - 'a']++; // don't do this
in[st.top() - 'a'] = false;
st.pop();
}
st.push(cc);
in[c] = true;
}
// ans
string ans = "";
while (st.size() > 0) {
ans += st.top();
st.pop();
}
reverse(ans.begin(), ans.end());
return ans;
}
};
#ifdef LOCAL
int main()
{
return 0;
}
#endif
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