[Leetcode] First Missing Positive

 1 public class Solution {
 2     public int firstMissingPositive(int[] A) {
 3         if(A==null||A.length==0)
 4             return 1;
 5         int n=A.length;
 6         for(int i=0;i<n;){
 7             if((A[i]!=i+1)&&A[i]>=1&&A[i]<=A.length&&A[A[i]-1]!=A[i]){
 8                 int temp=A[A[i]-1];
 9                 A[A[i]-1]=A[i];
10                 A[i]=temp;
11             }else{
12                 ++i;
13             }
14         }
15         for(int i=0;i<A.length;++i){
16             if(A[i]!=i+1)
17                 return i+1;
18         }
19         return A.length+1;
20     }
21 }