public int rob(int[] nums){
  if (nums == null || nums.length == 0)   return 0;
  if (nums.length == 1)   return nums[0];
  int preMax=nums[0];
  int max=Math.max(nums[0],nums[1]);
  for (int i=2; i < nums.length; i++) {
    int temp=preMax;
    preMax=max;
    max=Math.max(temp + nums[i],preMax);
  }
  return max;
}

public int rob ( int [ ] nums ) { if ( nums == null || nums . length == 0 ) return 0 ; if ( nums . length == 1 ) return nums [ 0 ] ; int preMax = nums [ 0 ] ; int max = Math . max ( nums [ 0 ] , nums [ 1 ] ) ; for ( int i = 2 ; i < nums . length ; i ++ ) { int temp = preMax ; preMax = max ; max = Math . max ( temp + nums [ i ] , preMax ) ; } return max ; }
recurrence relation : max [ n ] = max ( max [ n - 2 ] + nums [ n ] , max [ n - 1 ] ) optimization : use constant variables instead of an array
