寻找名人

https://leetcode.com/problems/find-the-celebrity/

Suppose you are at a party with n people (labeled from 0 to n - 1) and among them, there may exist one celebrity. The definition of a celebrity is that all the other n - 1people know him/her but he/she does not know any of them.

Now you want to find out who the celebrity is or verify that there is not one. The only thing you are allowed to do is to ask questions like: "Hi, A. Do you know B?" to get information of whether A knows B. You need to find out the celebrity (or verify there is not one) by asking as few questions as possible (in the asymptotic sense).

You are given a helper function bool knows(a, b) which tells you whether A knows B. Implement a function int findCelebrity(n), your function should minimize the number of calls to knows.

Note: There will be exactly one celebrity if he/she is in the party. Return the celebrity's label if there is a celebrity in the party. If there is no celebrity, return -1.

int findCelebrity(int n) {
    // 名人：其他人知道他、他不知道其他人
    int cand = 0;
    for (int i = 1; i < n; i++) {
        if (knows(cand, i)) { // cand知道i，cand不是名人
            cand = i;
        }
    }
    // [0..cand)知道其他人，[0..cand)不是名人；
    // [cand+1..n)不被cand知道，[cand+1..n)不是名人；
    // cand是唯一候选。下面验证cand是不是真名人。
    // 已知"cand不知道[cand+1..n)"（不用再验证）
    for (int i = 0; i < cand; i++) {
        if (!knows(i, cand) || knows(cand, i)) return -1;
    }
    for (int i = cand + 1; i < n; i++) {
        if (!knows(i, cand)) return -1;
    }
    return cand;
}