lintcode-medium-First Missing Positive

public class Solution {
    /**    
     * @param A: an array of integers
     * @return: an integer
     */
    public int firstMissingPositive(int[] A) {
        // write your code here    
        if(A == null || A.length <= 0)
            return 1;
        
        for(int i = 0; i < A.length; i++){
            
            while(A[i] > 0 && A[i] <= A.length && A[i] != i + 1){
                int temp = A[A[i] - 1];
                if(temp == A[i]){
                    break;
                }
                else{
                    A[A[i] - 1] = A[i];
                    A[i] = temp;
                }
            }
            
        }
        
        for(int i = 0; i < A.length; i++)
            if(A[i] != i + 1)
                return i + 1;
        
        return A.length + 1;
    }
}