Solution in O(1)

class Solution {
public:
    int rob(vector<int>& nums) {
        int n=nums.size();
        if(n==1)
            return nums[0];
        else if(n==2)
            return max(nums[0],nums[1]);
        int x,y,z;
        x=nums[0];
        y=max(nums[0],nums[1]);
        for(int i=2;i<n;i++)
        {
            z=max(y,nums[i]+x);
            x=y;
            y=z;
        }
        return z;
    }
};