First Missing Positive

 1 public class Solution {
 2     /**    
 3      * @param A: an array of integers
 4      * @return: an integer
 5      */
 6     public int firstMissingPositive(int[] A) {
 7         if (A == null || A.length == 0) return 1;
 8         
 9         for (int i = 0; i < A.length; i++) {
10             if (A[i] <= A.length && A[i] > 0 && A[A[i] - 1] != A[i]) {
11                exchange(A, A[i]);
12             }
13         }
14         
15         for (int i = 0; i < A.length; i++) {
16             if (A[i] != i + 1) {
17                 return i + 1;
18             }   
19         }
20         return A.length + 1;
21     }
22     
23     public void exchange(int[] A, int value) {
24         int temp = A[value - 1];
25         A[value - 1] = value;
26         if (temp <= A.length && temp > 0 && A[temp - 1] != temp) {
27             exchange(A, temp);
28         } 
29     }
30 }