[Leetcode] First Missing Positive

class Solution {
public:
int firstMissingPositive(int A[], int n) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
if (n < 0)
return 1;

for (int i = 0; i < n; ++i)
{
if (A[i] <= 0)
A[i] = n + 3;
}

for (int i = 0; i < n; ++i)
{
int cur;
A[i] < 0 ? cur = A[i] * (-1): cur = A[i];

if (cur <= n)
{
if (A[cur - 1] > 0)
{
A[cur - 1] *= -1;
}
}
}

for (int i = 0; i < n; ++i)
{
if (A[i] > 0)
return i + 1;
}

return n + 1;
}
};