src/main/java/com/fishercoder/solutions/_128.java

package com.fishercoder.solutions;

import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
import java.util.TreeSet;

public class _128 {
    public static class Solution1 {
        public int longestConsecutive(int[] nums) {
            Map<Integer, Integer> map = new HashMap();
            //<value, index>
            UnionFind uf = new UnionFind(nums);
            for (int i = 0; i < nums.length; i++) {
                if (map.containsKey(nums[i])) {
                    continue;
                }
                map.put(nums[i], i);
                if (map.containsKey(nums[i] - 1)) {
                    uf.union(i, map.get(nums[i] - 1));
                    //note: we want to union this index and nums[i]-1's root index which we can get from the map
                }
                if (map.containsKey(nums[i] + 1)) {
                    uf.union(i, map.get(nums[i] + 1));
                }
            }
            return uf.maxUnion();
        }

        class UnionFind {
            int[] ids;

            public UnionFind(int[] nums) {
                ids = new int[nums.length];
                for (int i = 0; i < nums.length; i++) {
                    ids[i] = i;
                }
            }

            public void union(int i, int j) {
                int x = find(ids, i);
                int y = find(ids, j);
                ids[x] = y;
            }

            public int find(int[] ids, int i) {
                while (i != ids[i]) {
                    ids[i] = ids[ids[i]];
                    i = ids[i];
                }
                return i;
            }

            public boolean connected(int i, int j) {
                return find(ids, i) == find(ids, j);
            }

            public int maxUnion() {
                //this is O(n)
                int max = 0;
                int[] count = new int[ids.length];
                for (int i = 0; i < ids.length; i++) {
                    count[find(ids, i)]++;
                    max = max < count[find(ids, i)] ? count[find(ids, i)] : max;
                }
                return max;
            }
        }
    }

    public static class Solution2 {
        //inspired by this solution: https://discuss.leetcode.com/topic/25493/simple-fast-java-solution-using-set
        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 max = 1;

            for (int num : nums) {
                if (set.remove(num)) {
                    int val = num;
                    int count = 1;
                    while (set.remove(val - 1)) {
                        val--;//we find all numbers that are smaller than num and remove them from the set
                    }
                    count += num - val;

                    val = num;
                    while (set.remove(val + 1)) {
                        val++;//then we find all numbers that are bigger than num and also remove them from the set
                    }
                    count += val - num;

                    max = Math.max(max, count);
                }
            }
            return max;
        }
    }

    public static class Solution3 {
        /**
         * O(n) time complexity.
         */
        public int longestConsecutive(int[] nums) {
            Set<Integer> set = new HashSet<>();
            for (int num : nums) {
                set.add(num);
            }

            int longestStreak = 0;
            for (int num : set) {
                //we'll go through this set instead of nums, this makes a big difference in time complexity, esp. based on LeetCode test cases
                if (!set.contains(num - 1)) {
                    int currentNum = num;
                    int currentStreak = 1;

                    while (set.contains(currentNum + 1)) {
                        currentNum += 1;
                        currentStreak += 1;
                    }
                    longestStreak = Math.max(longestStreak, currentStreak);
                }
            }
            return longestStreak;
        }
    }

    public static class Solution4 {
        /**
         * O(nlogn) time complexity
         */
        public int longestConsecutive(int[] nums) {
            if (nums.length == 0) {
                return 0;
            }
            TreeSet<Integer> treeSet = new TreeSet<>();
            for (int i : nums) {
                treeSet.add(i);//O(logn) time complexity for each add() call
            }
            int ans = 1;
            Iterator<Integer> it = treeSet.iterator();
            Integer curr = it.next();
            int len = 1;
            while (it.hasNext()) {
                Integer next = it.next();
                if (curr + 1 == next) {
                    len++;
                } else {
                    len = 1;
                }
                curr = next;
                ans = Math.max(ans, len);
            }
            ans = Math.max(ans, len);
            return ans;
        }
    }
}