FindTheCelebrity.java

import java.util.*;

public class FindTheCelebrity {
    static int N = 7;
    static Map<Integer, List<Integer>> relation = new HashMap();
    static {
        relation.put(0, Arrays.asList(1));
        relation.put(1, new ArrayList());  // celebrity
        relation.put(2, Arrays.asList(1));
        relation.put(3, Arrays.asList(1, 2));
        relation.put(4, Arrays.asList(1));
        relation.put(5, Arrays.asList(1));
        relation.put(6, Arrays.asList(1, 5));
    }

    static boolean knows(int a, int b) {
        return relation.get(a).contains(b);
    }

    static int findCelebrity() {
        Stack<Integer> stack = new Stack();
        for (int i = 0; i < N; i++) {
            stack.push(i);
        }

        // elimination
        while (stack.size() > 1) {
            int a = stack.pop();
            int b = stack.pop();
            if (knows(a, b)) {
                // a knows b
                // b is a potential celeb
                // make b back to stack
                stack.push(b);
            } else {
                // a doesn't knows b
                // a is a potential celeb
                // make a back to stack
                stack.push(a);
            }
        }
        int c = stack.pop();

        // verification
        for (int i = 0; i < N; i++) {
            if ((i != c) && (knows(c, i) || !knows(i, c))) {
                return -1;
            }
        }
        return c;
    }

    public static void main(String[] args) {
        System.out.println(findCelebrity());
    }
}