LeetCode--First Missing Positive

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;    }};