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class Solution {
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 public:
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  stack<char> st;
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  string removeDuplicates(string s) {
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    string ans;
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    char c;
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    for (auto &i : s) {
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      if (st.empty()) {
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        st.push(i);
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        continue;
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      }
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      if (i == st.top())
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        st.pop();
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      else
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        st.push(i);
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    }
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    while (!st.empty()) {
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      c = st.top();
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      ans += c;
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      st.pop();
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    }
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    reverse(begin(ans), end(ans));
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    return ans;
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  }
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};