Another from the archives. I think this is a nice introduction to a more “stateful” complicated loop. A productive approach was to break it into cases. I think this is a nice introduction to state machines.

class Solution {
    public:
    enum class dir {
        UNKNOWN,
        INCREASING,
        DECREASING
    };
    bool isMonotonic(vector<int>& nums) {
        auto state = dir::UNKNOWN;
        for (int i = 1; i < nums.size(); i++) {
            switch (state) {
                case dir::UNKNOWN:
                    if (nums[i-1] < nums[i]) {
                        state = dir::INCREASING;
                    }
                    else if (nums[i-1] > nums[i]) {
                        state = dir::DECREASING;
                    }
                    break;
                case dir::INCREASING:
                    if (nums[i-1] > nums[i]) {
                        return false;
                    }
                    break;
                case dir::DECREASING:
                    if (nums[i-1] < nums[i]) {
                        return false;
                    }
                    break;
            }
        }
        return true;
    }
};