1 public class Solution {
 2     public int firstMissingPositive(int[] A) {
 3         if(A.length<=0) return 1;
 4         int n = A.length;
 5         for(int i=0;i<n;i++){
 6             if(A[i]<=0){
 7                 A[i]=n+2;
 8             }
 9         }
10         for(int i=0;i<n;i++){
11             int temp = Math.abs(A[i]);
12             if(temp<=n){
13                 A[temp-1] = -Math.abs(A[temp-1]);
14             }
15         }
16         for(int i=0;i<n;i++){
17             if(A[i]>0){
18                 return i+1;
19             }
20         }
21         return n+1;
22     }
23 }