find celebrity: done

Author: rampatra

src/main/java/com/leetcode/arrays/FindTheCelebrity.java

@@ -0,0 +1,105 @@
+package com.leetcode.arrays;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * Level: Medium
+ * Problem Link: https://leetcode.com/problems/find-the-celebrity/
+ * Problem Description:
+ * 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 - 1 people 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). 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.
+ *
+ * Example 1:
+ *
+ * Input: graph = [
+ *   [1,1,0],
+ *   [0,1,0],
+ *   [1,1,1]
+ * ]
+ * Output: 1
+ * Explanation: There are three persons labeled with 0, 1 and 2. graph[i][j] = 1 means person i knows person j, otherwise
+ * graph[i][j] = 0 means person i does not know person j. The celebrity is the person labeled as 1 because both 0 and 2
+ * know him but 1 does not know anybody.
+ *
+ *
+ * Example 2:
+ *
+ * Input: graph = [
+ *   [1,0,1],
+ *   [1,1,0],
+ *   [0,1,1]
+ * ]
+ * Output: -1
+ * Explanation: There is no celebrity.
+ *
+ *
+ * Note: The directed graph is represented as an adjacency matrix, which is an n x n matrix where a[i][j] = 1 means
+ * person i knows person j while a[i][j] = 0 means the contrary. Remember that you won't have direct access to the
+ * adjacency matrix.
+ *
+ * @author rampatra
+ * @since 2019-08-04
+ */
+public class FindTheCelebrity {
+
+    private int[][] knowsMatrix;
+
+    FindTheCelebrity(int[][] knowsMatrix) {
+        this.knowsMatrix = knowsMatrix;
+    }
+
+    public boolean knows(int a, int b) {
+        return knowsMatrix[a][b] == 1;
+    }
+
+    /**
+     * Time Complexity: O(n)
+     * Space Complexity: O(1)
+     * Runtime: <a href="https://leetcode.com/submissions/detail/249123409/">6 ms</a>.
+     *
+     * @param n
+     * @return
+     */
+    public int findCelebrity(int n) {
+        int celebrityIndex = 0;
+
+        for (int i = 1; i < n; i++) {
+            // if a person doesn't know another person then he maybe a celebrity
+            if (!knows(i, celebrityIndex)) {
+                celebrityIndex = i;
+            }
+        }
+
+        for (int i = 0; i < n; i++) {
+            // verify whether the celebrity only knows himself and all other people know the celebrity
+            if ((knows(celebrityIndex, i) && i != celebrityIndex) || !knows(i, celebrityIndex)) {
+                return -1;
+            }
+        }
+
+        return celebrityIndex;
+    }
+
+    public static void main(String[] args) {
+        FindTheCelebrity findTheCelebrity = new FindTheCelebrity(new int[][]{
+                {1, 1, 0},
+                {0, 1, 0},
+                {1, 1, 1}});
+
+        assertEquals(1, findTheCelebrity.findCelebrity(3));
+
+        findTheCelebrity = new FindTheCelebrity(new int[][]{
+                {1, 0},
+                {0, 1}});
+
+        assertEquals(-1, findTheCelebrity.findCelebrity(2));
+    }
+}