Given an unsorted integer array, find the first missing positive integer.

For example,
Given [1,2,0] return 3,
and [3,4,-1,1] return 2.

Your algorithm should run in O(n) time and uses constant space.

class Solution 
{
public:
    int firstMissingPositive(int A[], int n) 
    {
        if(n==0)
            return 1;
        bool* flag  = new bool[n];
        for(int i=0; i<n; i++)
            flag[i] = false;
        for(int i=0; i<n; i++)
            if(A[i]>0)
                flag[A[i]-1] = true;
        int i = 0;
        for(i=0; i<n; i++)
            if(flag[i] == false)
                break;
        delete[] flag;
        return i+1;
        /*
        if(n==0)
            return 1;
        for(int i=0; i<n;)
        {
            if(A[i]!=i+1 && A[i]>0 && A[i]<=n && A[A[i]-1] != A[i])
                swap(A,i,A[i]-1);
            else
                i++;
        }
        for(int i=0; i<n; i++)
            if(A[i] != i+1)
                return i+1;
        return n+1;
        */
    }
    void swap(int* a, int i, int j)
    {
        int t = a[i];
        a[i] = a[j];
        a[j] = t;
    }
};

LeetCode--First Missing Positive