128. Longest Consecutive Sequence.md

128. Longest Consecutive Sequence

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Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

Your algorithm should run in O(n) complexity.

Example:

Input: [100, 4, 200, 1, 3, 2]
Output: 4
Explanation: The longest consecutive elements sequence is [1, 2, 3, 4]. Therefore its length is 4.

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Solution

• mine
  • Java
    • Arrays.sort Runtime: 2 ms, faster than 100.00%, Memory Usage: 40.3 MB, less than 34.83% of Java online submissions

      //O(N * logN)time O(N)space
      public int longestConsecutive(int[] nums) {
          if (nums == null || nums.length == 0) {
              return 0;
          }
          Arrays.sort(nums);
          int t = 1;
          int res = 1;
          for (int i = 1; i < nums.length; i++) {
              if (nums[i] - nums[i - 1] <= 1) {
                  t += nums[i] - nums[i - 1];
              } else {
                  res = Math.max(res, t);
                  t = 1;
              }
          }
          res = Math.max(res, t);
          return res;
      }
      

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• the most votes

  • HashMap Runtime: 5 ms, faster than 49.11%, Memory Usage: 40 MB, less than 45.16% of Java online submissions

    //O(N)time O(N)space
    public int longestConsecutive(int[] num) {
        int res = 0;
        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
        for (int n : num) {
            if (!map.containsKey(n)) {
                int left = (map.containsKey(n - 1)) ? map.get(n - 1) : 0;
                int right = (map.containsKey(n + 1)) ? map.get(n + 1) : 0;
                // sum: length of the sequence n is in
                int sum = left + right + 1;
                map.put(n, sum);
    
                // keep track of the max length 
                res = Math.max(res, sum);
    
                // extend the length to the boundary(s)
                // of the sequence
                // will do nothing if n has no neighbors
                map.put(n - left, sum);
                map.put(n + right, sum);
            }
        }
        return res;
    }
    

  • HashSet Runtime: 3 ms, faster than 91.81%, Memory Usage: 39.6 MB, less than 75.77% of Java online submissions

    //O(N)time O(N)space
    public int longestConsecutive(int[] nums) {
        if (nums == null || nums.length == 0) return 0;
        Set<Integer> set = new HashSet<>();
        for (int i : nums) set.add(i);
        int ans = 0;
        for (int num : nums) {
            int left = num - 1;
            int right = num + 1;
            set.remove(num);
            while (set.remove(left)) left--;
            while (set.remove(right)) right++;
            ans = Math.max(ans, right - left - 1);
            if (set.isEmpty()) return ans;
        }
        return ans;
    }