For every house k, there are two options: either to rob it (include this house: i) or not rob it (exclude this house: e).

Include this house:
i = num[k] + e (money of this house + money robbed excluding the previous house) 
Exclude this house:
e = max(i, e) (max of money robbed including the previous house or money robbed excluding the previous house)
(note that i and e of the previous step, that's why we use tmp here to store the previous i when calculating e, to make O(1) space)
Here is the code:
public class Solution {
public int rob(int[] num) {
int i = 0;
int e = 0;
for (int k = 0; k<num.length; k++) {
int tmp = i;
i = num[k] + e;
e = Math.max(tmp, e);
}
return Math.max(i,e);
}
}