Step 2: Route on the Abstract Graph (The "Highway" Part):
Less Than (2): Everything in this space must be less than 2. The answer is 6-0, placed horizontally.
，更多细节参见搜狗输入法2026
// 栈不为空时才判断（避免访问stack.at(-1)时报错）
// 注意：slice(0, 负数)会返回空字符串，需兼容（比如stack长度<k时，slice后为空）