Avoid Flood in The City LeetCode solution in C++

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class Solution {
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 public:
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  vector<int> avoidFlood(vector<int> &rains) {
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    vector<int> ans(rains.size(), -1);
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    unordered_map<int, int> indices; // store the lake and its index
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    set<int> st;                     // stores the indices of zeros
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    for (int i = 0; i < rains.size(); ++i) {
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      if (!rains[i])
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        st.insert(i);
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      else {
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        if (indices.find(rains[i]) == end(indices))
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          indices[rains[i]] = i;
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        else {
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          int prevDay = indices[rains[i]];
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          auto it = st.upper_bound(prevDay);
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          if (it == end(st)) return {};
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          ans[*it] = rains[i];
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          indices[rains[i]] = i;
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          st.erase(it);
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        }
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      }
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    }
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    for (int i = 0; i < ans.size(); ++i) {
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      if (!rains[i] and ans[i] == -1) ans[i] = 1;
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    }
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    return ans;
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  }
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};