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;
    }
};