C++:
class Solution {
public:
    int firstMissingPositive(int A[], int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        for(int i=0;i<n;i++){
            if(A[i]>0&&A[i]<=n){
                if(A[i]-1!=i&&A[A[i]-1]!=A[i]){
                    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;
    }
};
Java:
public class Solution {
    public int firstMissingPositive(int[] A) {
        // Start typing your Java solution below
        // DO NOT write main() function
        int n=A.length;
        for(int i=0;i<n;i++){
            if(A[i]>0&&A[i]<=n){
                if(A[i]-1!=i&&A[A[i]-1]!=A[i]){
                    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;
    }
}