1 public class Solution{    
 2     public int firstMissingPositive(int[] A){
 3         int result = 1; //if the array is empty, we result 1
 4         if(A.length > 0){
 5             for(int i = 0; i < A.length; ++i){
 6                 //do not forget the condition A[A[i] - 1] != A[i];
 7                 //otherwise, there is a dead loop on input, for example, [2,2]
 8                 if(A[i] > 0 && A[i] <= A.length && A[i] != i + 1 && A[A[i] - 1] != A[i]){
 9                     int temp = A[i];
10                     int temp1 = A[A[i] - 1];
11                     A[A[i] - 1] = A[i];
12                     A[i] = temp1;
13                     i = (temp > i + 1) ? i - 1 : i;
14                 }
15             }
16             for(int i = 0; i < A.length; ++i){
17                 if(A[i] != i + 1){
18                     result = i + 1;
19                     break;
20                 }
21                 else
22                     result = A.length + 1;
23             }
24         }
25         return result;
26     }
27 }