public class Solution {

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); // no noeed to care elements in middle because we never need to use it agian.

map.put(n + right, sum);

}

}

return res;

}

}