Longest Consecutive Sequence

问题解法1：

We will use HashMap. The key thing is to keep track of the sequence length and store that in the boundary points of the sequence. For example, as a result, for sequence {1, 2, 3, 4, 5}, map.get(1) and map.get(5) should both return 5.

Whenever a new element n is inserted into the map, do two things:

1. See if n - 1 and n + 1 exist in the map, and if so, it means there is an existing sequence next to n. Variables left and right will be the length of those two sequences, while 0means there is no sequence and n will be the boundary point later. Store (left + right + 1) as the associated value to key n into the map.
2. Use left and right to locate the other end of the sequences to the left and right of n respectively, and replace the value with the new length.

Everything inside the for loop is O(1) so the total time is O(n). Please comment if you see something wrong. Thanks.

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;}

用python 实现的相同代码：

class Solution(object):    def longestConsecutive(self, nums):        """        :type nums: List[int]        :rtype: int        """        dict ={}        res = 0        for num in nums:            if dict.get(num) == None:                left = dict.get(num - 1)                right = dict.get(num + 1)                if left == None:                    left = 0                if right == None:                    right = 0                sum = left + right + 1                dict[num] = sum;                dict[num - left] = sum                dict[num + right] = sum                res = max(res, sum)        return res;