LeetCode/java/longest-consecutive-sequence.java

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;
  }
}