Minimum Consecutive Cards to Pick Up LeetCode solution in C++

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class Solution {
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 public:
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  int minimumCardPickup(vector<int> &cards) {
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    int res(INT_MAX), n(size(cards));
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    unordered_map<int, int> m;
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    for (auto i = 0; i < n; i++) {
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      // number of consecutive cards you have to pick up to have a pair of
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      // matching cards == (Diference between 2 indexes of same card) + 1
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      if (m.count(cards[i])) res = min(res, i - m[cards[i]] + 1);
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      m[cards[i]] = i;
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    }
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    return (res == INT_MAX) ? -1 : res;
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  }
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};