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