First Missing Positive

Given an unsorted integer array, find the first missing positive integer.
For example,
Given [1,2,0] return 3,
and [3,4,-1,1] return 2.
Your algorithm should run in O(n) time and uses constant space.

Naive Way: Constant space here is a big constrain. The key here is by swapping. Since the length of the array is fixed. There can be at most array.length positive integers. Find the correct position for each valid positive integer by swapping. That will achieve constant space.

 public class Solution {  
   public int firstMissingPositive(int[] A) {  
     int i = 0;  
     while(i < A.length){  
       if(A[i] == i+1 || A[i] <= 0 || A[i] > A.length) i++;  
       else if(A[A[i]-1] != A[i]) swap(A, i, A[i]-1);  
       else i++;  
     }  
     i = 0;  
     while(i < A.length && A[i] == i+1) i++;  
     return i+1;  
   }  
   private void swap(int[] A, int i, int j){  
     int temp = A[i];  
     A[i] = A[j];  
     A[j] = temp;  
   }  
 }