Longest Consecutive Sequence

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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.

1. HashMap. 时间: O(N) 空间: O(N)

2. sort + search. 时间: O(NlgN) 空间：O(1) ***注意sort后前后两个元素相等的情况！！

public class Solution {    public int longestConsecutive(int[] num) {        //map num[i] to 0/1 (represent that if it has been visited)        HashMap<Integer, Integer> map = new HashMap<>();        for (int i : num) {            map.put(i, 0);        }                int max = 1;                for (int i : num) {            if (map.get(i) == 1) continue;                        int temp = i;            int thismax = 1;                        while (map.containsKey(temp + 1)) {                thismax++;                temp++;                map.put(temp, 1);            }                        temp = i;            while (map.containsKey(temp - 1)) {                thismax++;                temp--;                map.put(temp, 1);            }                        max = Math.max(max, thismax);        }        return max;    }}

public class Solution {    public int longestConsecutive(int[] num) {       Arrays.sort(num);       int max = 1;       int thismax = 1;       for (int i = 1; i < num.length; i++) {           if (num[i] == num[i-1]) continue;           if (num[i]-1 == num[i-1]) {               thismax++;                              max = Math.max(max, thismax);           } else {               thismax = 1;           }       }       return max;    }}

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