连续 + 两端异号 ⇒ 有零点；个数要结合单调性判。
Consider our voting example from Section 1.2: three candidates A, B, and C are running for office. We decided that A …
甲和乙赢面一样大，丙只有他俩一半——然后甲退选了。按比例把甲的票重新分给乙和丙，答案干净得 2/3 和 1/3。
