Find Celebrity

Author: JoshuaJiangFD

src/main/java/joshua/leetcode/array/FindCelebrity.java

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+package joshua.leetcode.array;
+
+import java.util.LinkedList;
+import java.util.List;
+
+/**
+ * 277. Find the Celebrity<br/>
+ * <p/>
+ * <a href="https://leetcode.com/problems/find-the-celebrity/">leetcode link</a>
+ *
+ * @author Joshua.Jiang on 2016/5/22.
+ */
+public abstract class FindCelebrity {
+
+    /**
+     * Suppose you are at a party with n people (labeled from 0 to n - 1) and among them, there may exist one celebrity.
+     * <p/>
+     * The definition of a celebrity is that all the other n - 1 people know him/her but he/she does not know any of them.
+     * <p/>
+     * Now you want to find out who the celebrity is or verify that there is not one.
+     * <p/>
+     * 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.
+     * <p/>
+     * You need to find out the celebrity (or verify there is not one) by asking as few questions as possible (in the asymptotic sense).
+     * <p/>
+     * 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.
+     *
+     * @param n the number of persons
+     * @return -1 if no celebrity or the label of celebrity
+     */
+    public abstract int findCelebrity(int n);
+
+    /**
+     *
+     * @param a
+     * @param b
+     * @return true if a knows b else false.
+     */
+    public abstract boolean knows(int a, int b);
+
+    public abstract static class Solution1 extends FindCelebrity {
+
+        @Override
+        public int findCelebrity(int n) {
+            // the person to search is initially the persons whom person with label 0 knows.
+            // the this list must contains celebrity
+            List<Integer> personsToSearch = new LinkedList<Integer>();
+            for(int i = 0; i < n; i++) {
+                personsToSearch.add(i);
+            }
+            if (personsToSearch.size() == 0) {
+                return -1;
+            }
+            if (personsToSearch.size() == 1) {
+                return 0;
+            }
+            int celebrity = -1;
+            //everytime shrink the size of persons to search
+            while(true) {
+                List<Integer> temp = new LinkedList<Integer>();
+                for (int i =1;  i < personsToSearch.size(); i++) {
+                    if(knows(personsToSearch.get(0), personsToSearch.get(i))) {
+                        temp.add(personsToSearch.get(i));
+                    }
+                }
+                if (temp.size() == 0) {
+                    // personsToSearch.get(0) is likely to be a celebrity, 'cause he know no one else in the personToSearch
+                    // list
+                    celebrity = personsToSearch.get(0);
+                    break;
+                }
+                if (temp.size() == 1) {
+                    //  temp.get(0) is likely to be a celebrity, 'cause the first person in personToSearch list only know
+                    // this guy
+                    celebrity = temp.get(0);
+                    break;
+                }
+                personsToSearch = temp;
+            }
+            // validate if the celebrity is true
+            for (int i = 0; i < n; i++) {
+                if (i != celebrity) {
+                    if (knows(celebrity, i)) {
+                        return -1;
+                    }
+                    if (!knows(i, celebrity)) {
+                        return -1;
+                    }
+                }
+            }
+            return celebrity;
+        }
+    }
+}

src/main/java/joshua/leetcode/solutiontag/TwoPointers.java

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+package joshua.leetcode.solutiontag;
+
+/**
+ * @author Joshua.Jiang on 2016/5/22.
+ */
+public @interface TwoPointers {
+}