Solution for Longest Consecutive Sequence #240

Author: dohyeon2

longest-consecutive-sequence/dohyeon2.java

@@ -0,0 +1,79 @@
+import java.util.HashSet;
+
+class Solution {
+    // This solution was inspired by:
+    // https://www.algodale.com/problems/longest-consecutive-sequence/
+    //
+    // I initially believed this algorithm would run in O(n) time,
+    // but it resulted in a Time Limit Exceeded error.
+    //
+    // Although the expected time complexity is O(n),
+    // repeatedly calling set.iterator().next() might introduce overhead.
+    // Iterating through the set using a for-loop may be a better approach.
+    //
+    // In this case, it seems preferable to follow the approach described here:
+    // https://www.algodale.com/problems/longest-consecutive-sequence/#%ED%92%80%EC%9D%B4-3
+
+    public int oldApproach(int[] nums) {
+        int count = 0;
+        HashSet<Integer> set = new HashSet<>();
+        for (int num : nums) {
+            set.add(num);
+        }
+
+        while (set.size() > 0) {
+            int buffer = 1;
+            // This may cause a Time Limit Exceeded error.
+            Integer curr = set.iterator().next();
+            set.remove(curr);
+            Integer next = curr + 1;
+            Integer prev = curr - 1;
+
+            while (set.contains(next)) {
+                set.remove(next);
+                next++;
+                buffer++;
+            }
+
+            while (set.contains(prev)) {
+                set.remove(prev);
+                prev--;
+                buffer++;
+            }
+
+            count = Math.max(count, buffer);
+        }
+
+        return count;
+    }
+
+    public int longestConsecutive(int[] nums) {
+        int count = 0;
+
+        // The Set has O(n) space complexity,
+        // because it may store up to n elements in memory.
+        // Is this the correct way to evaluate space complexity?
+        HashSet<Integer> set = new HashSet<>();
+        for (int num : nums) {
+            set.add(num);
+        }
+
+        for (int num : set) {
+            if (set.contains(num - 1)) {
+                continue;
+            }
+
+            int currentNum = num;
+            int currentCount = 1;
+
+            while (set.contains(currentNum + 1)) {
+                currentNum++;
+                currentCount++;
+            }
+
+            count = Math.max(count, currentCount);
+        }
+
+        return count;
+    }
+}