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

Round 2;

class Solution {
public:
int firstMissingPositive(int A[], int n) {
if(n == 0)
return 1;
for(int i = 0; i < n; i++)
{
if(A[i] <= 0 || A[i] > n || A[i] == i+1 || A[A[i]-1] == A[i])
continue;
else
{
int temp = A[A[i]-1];
A[A[i]-1] = A[i];
A[i] = temp;
i--;
}
}
for(int i = 0; i < n; i++)
{
if(A[i] != i+1)
return i+1;
}
return n+1;
}
};