[LintCode] Longest Consecutive Sequence 求最长连续序列

Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

Have you met this question in a real interview? 

Yes

Clarification

Your algorithm should run in O(n) complexity.

Example

Given [100, 4, 200, 1, 3, 2],
The longest consecutive elements sequence is[1, 2, 3, 4]. Return its length: 4.

LeetCode上的原题，请参见我之前的博客Longest Consecutive Sequence。

解法一：

class Solution {
public:
    /**
     * @param nums: A list of integers
     * @return an integer
     */
    int longestConsecutive(vector<int> &num) {
        int res = 0;
        unordered_set<int> s(num.begin(), num.end());
        for (int d : num) {
            if (!s.count(d)) continue;
            s.erase(d);
            int pre = d - 1, next = d + 1;
            while (s.count(pre)) s.erase(pre--);
            while (s.count(next)) s.erase(next++);
            res = max(res, next - pre - 1);
        }
        return res;
    }
};

解法二：

class Solution {
public:
    /**
     * @param nums: A list of integers
     * @return an integer
     */
    int longestConsecutive(vector<int> &num) {
        int res = 0;
        unordered_map<int, int> m;
        for (int d : num) {
            if (m.count(d)) continue;
            int left = m.count(d - 1) ? m[d - 1] : 0;
            int right = m.count(d + 1) ? m[d + 1] : 0;
            int sum = left + right + 1;
            m[d] = sum;
            res = max(res, sum);
            m[d - left] = sum;
            m[d + right] = sum;
        }
        return res;
    }
};