Longest Consecutive Sequence: Accepted

Author: gouthampradhan

README.md

@@ -80,6 +80,7 @@ My accepted leetcode solutions to some of the common interview problems.
 - [Course Schedule II](problems/src/depth_first_search/CourseScheduleII.java) (Medium)
 - [Alien Dictionary](problems/src/depth_first_search/AlienDictionary.java) (Hard)
 - [Graph Valid Tree](problems/src/depth_first_search/GraphValidTree.java) (Medium)
+- [Longest Consecutive Sequence](problems/src/depth_first_search/LongestConsecutiveSequence.java) (Medium)
 
 #### [Design](problems/src/design)
 

problems/src/depth_first_search/LongestConsecutiveSequence.java

@@ -0,0 +1,74 @@
+package depth_first_search;
+
+import java.util.*;
+
+/**
+ * Created by gouthamvidyapradhan on 15/12/2017.
+ * Given an unsorted array of integers, find the length of the longest consecutive elements sequence.
+
+ For example,
+ Given [100, 4, 200, 1, 3, 2],
+ The longest consecutive elements sequence is [1, 2, 3, 4]. Return its length: 4.
+
+ Your algorithm should run in O(n) complexity.
+
+ Solution: O(n) time and space complexity - Build a graph linking each number which is greater or lesser by one.
+ Perform a dfs to count the depth of a graph.
+
+ Dfs using recursion fails due to StackOverFlowError(due to deep recursion) hence used a iterative approach with a
+ stack
+ */
+public class LongestConsecutiveSequence {
+
+    private Map<Integer, Set<Integer>> graph;
+    private Set<Integer> done;
+    /**
+     * Main method
+     * @param args
+     * @throws Exception
+     */
+    public static void main(String[] args) throws Exception{
+        int[] nums = {-1, 0, -3, -2, 1, 2, 3, 4, 5, 4};
+        System.out.println(new LongestConsecutiveSequence().longestConsecutive(nums));
+    }
+
+    public int longestConsecutive(int[] nums) {
+        done = new HashSet<>();
+        graph = new HashMap<>();
+        for (int u : nums) {
+            graph.putIfAbsent(u, new HashSet<>());
+            if (graph.keySet().contains(u - 1)) {
+                graph.get(u - 1).add(u);
+                graph.get(u).add(u - 1);
+            }
+            if (graph.keySet().contains(u + 1)) {
+                graph.get(u + 1).add(u);
+                graph.get(u).add(u + 1);
+            }
+        }
+        int max = 0;
+        for(int i : graph.keySet()){
+            if(!done.contains(i)){
+                Stack<Integer> stack = new Stack<>();
+                stack.add(i);
+                max = Math.max(max, dfs(0, stack));
+            }
+        }
+        return max;
+    }
+
+
+    private int dfs(int count, Stack<Integer> stack){
+        while(!stack.isEmpty()){
+            int top = stack.pop();
+            count++;
+            done.add(top);
+
+            Set<Integer> children =  graph.get(top);
+            if(children != null){
+                children.stream().filter(c -> !done.contains(c)).forEach(stack::push);
+            }
+        }
+        return count;
+    }
+}