Solution 1 simulation

• t-complexity: $O(n + C)$ C is size of charset

• s-complexity: $O(C)$

直接暴力会超时

• s 和 goal 长度或者词频不同，则不为 buddy string

• s 与 goal 不同的字符数量为 2 （能相互交换）

• s and goal diff char number is 0, and s has char which appear more than twice