LeetCode "First Missing Positive"

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Similar with "Longest Consecutive Sequence". Another usage to hashset.

Take care of corner cases!

class Solution {public:    int firstMissingPositive(int A[], int n) {        if (n == 0) return 1;        unordered_set<int> set;        int minPos = std::numeric_limits<int>::max();        for (int i = 0; i < n; i ++)        {            int n = A[i];            if (n >= 0)            {                set.insert(n);                minPos = std::min(minPos, n);            }        }        if (minPos > 1) return 1;        int p = std::numeric_limits<int>::max();        if (set.size() == 0) return 1;        for (int i = 0; i < n; i++)        {            int n = A[i];            if (n < 0) continue;            int n1 = n - 1;            int n2 = n + 1;            if (n1 > 0)            {                if (set.find(n1) == set.end())                    p = std::min(p, n1);            }            if (n2 > 0)            {                if (set.find(n2) == set.end())                    p = std::min(p, n2);            }        }        return p;    }};