java: add «128. Longest Consecutive Sequence»

Signed-off-by: Matej Focko <me@mfocko.xyz>

java/longest-consecutive-sequence.java

@@ -0,0 +1,33 @@
+import java.util.HashMap;
+
+class Solution {
+  public int longestConsecutive(int[] nums) {
+    var uniqueNums = new HashMap<Integer, Boolean>();
+    for (var num : nums) {
+      uniqueNums.put(num, false);
+    }
+
+    // mark the possible starts
+    for (var num : nums) {
+      if (!uniqueNums.containsKey(num - 1)) {
+        uniqueNums.put(num, true);
+      }
+    }
+
+    var longest = 0;
+    for (var num : uniqueNums.entrySet()) {
+      if (!num.getValue()) {
+        continue;
+      }
+
+      var length = 1;
+      for (int x = num.getKey() + 1; uniqueNums.containsKey(x); ++x) {
+        ++length;
+      }
+
+      longest = Math.max(longest, length);
+    }
+
+    return longest;
+  }
+}