House Robber

You are a professional robber planning to rob houses along a street. Each house has a certain amount of money stashed, the only constraint stopping you from robbing each of them is that adjacent houses have security system connected and it will automatically contact the police if two adjacent houses were broken into on the same night.
Given a list of non-negative integers representing the amount of money of each house, determine the maximum amount of money you can rob tonight without alerting the police.

Naive Thinking: First thought was to use DP. The logic is as follows:
opt[i] // the maximum amount of money one rob without alerting the police.
// base case
opt[0] = 0
opt[1] = num[0]
// iteration
opt[i] = max(opt[i-1], opt[i-2]+num[i-1])

 public class Solution {  
   public int rob(int[] num) {  
     if(num == null || num.length == 0) return 0;  
     int opt[] = new int[num.length+1];  
     // base case  
     opt[0] = 0;  
     opt[1] = num[0];  
     // iteration  
     for(int i = 2;i <= num.length;i++)  
       opt[i] = Math.max(opt[i-1], opt[i-2] + num[i-1]);  
     return opt[num.length];  
   }  
 }  

From the structure of the code. It is easy to see the O(N) space could be constrained to O(1).

 public class Solution {  
   public int rob(int[] num) {  
     if(num == null || num.length == 0) return 0;  
     // base case  
     int pre = 0;  
     int cur = num[0];  
     // iteration  
     for(int i = 2;i <= num.length;i++){  
       int temp = cur;  
       cur = Math.max(cur, pre + num[i-1]);  
       pre = temp;  
     }  
     return cur;  
   }  
 }