128_LongestConsecutiveSequence.java

/*
 * Given an unsorted array of integers, find the length 
 * of the longest consecutive elements sequence.
 * For example,
 * Given [100, 4, 200, 1, 3, 2],
 * The longest consecutive elements sequence is 
 * [1, 2, 3, 4]. Return its length: 4.
 * Your algorithm should run in O(n) complexity.
 */


public class Solution {
    public int longestConsecutive(int[] num) {
        Set<Integer> set = new HashSet<Integer>();
        int res = 0;
        for(int value : num) {
            set.add(value);
        }
        for(int value : num) {
            int low = value;
            int high = value + 1;
            while(set.contains(low - 1)) set.remove(low--);
            while(set.contains(high)) set.remove(high++);
            res = Math.max(res, high - low);
        }
        return res;
    }
}

//Many other algorithm
https://github.com/mitcc/AlgoSolutions/blob/master/leetcode/LongestConsecutiveSequence.java

//Check it
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);
        }
        else {
            // duplicates
            continue;
        }
    }
    return res;
}