Leetcode/128.LongestConsecutiveSequence.h at master · bluepp/Leetcode · GitHub

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/*
    bluepp

    2014-06-12
    2014-07-15
    2014-12-10
    2014-12-22

    May the force be with me!

    Problem:    Longest Consecutive Sequence
    Source:     https://oj.leetcode.com/problems/longest-consecutive-sequence/
    Notes:
    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.

    Solution: Update solution. This solution is from Peking2 (http://blog.sina.com.cn/s/blog_b9285de20101iqar.html).
              This solution is much easier to understand.
*/

   int longestConsecutive(vector<int>& nums) {
        int ret = 0;
        unordered_set<int> set(nums.begin(), nums.end());

        for (auto p : nums) {
            if (!set.count(p)) {
                continue;
            }

            set.erase(p);
            int prev = p-1, next = p+1;
            while (set.count(prev)) {
                set.erase(prev);
                prev--;
            }
            while (set.count(next)) {
                set.erase(next);
                next++;
            }

            ret = max(ret, next-prev-1);
        }

        return ret;

    }


/* another one */
    int longestConsecutive(vector<int>& nums) {

        int ret = 0;
        unordered_map<int, int> map;

        for (auto p : nums)
        {
            if (!map.count(p))
            {
                int l = map.count(p-1) ? map[p-1] : 0;
                int r = map.count(p+1) ? map[p+1] : 0;

                int sum = l+r+1;
                map[p] = sum;
                ret = max(ret, sum);

                map[p-l] = sum;
                map[p+r] = sum;
            }
        }

        return ret;

    }


/* 2016-06-26 , unordered_map */
   int longestConsecutive(vector<int>& nums) {

        int ret = 0;
        unordered_map<int, int> map;


        for (int i = 0; i < nums.size(); i++)
        {
            map[nums[i]] = i;
        }

        for (int i = 0; i < nums.size(); i++)
        {
            int d = nums[i];
            int n = 1;

            map.erase(d);
            while (map.find(++d) != map.end())
            {
                n++;
                map.erase(d);
            }

            d = nums[i];
            while (map.find(--d) != map.end())
            {
                n++;
                map.erase(d);
            }

            ret = max(ret, n);
        }

        return ret;
    }


    int longestConsecutive(vector<int> &num) {
        unordered_set<int> s;
        int n = num.size();

        for (int i = 0; i < n; i++)
            s.insert(num[i]);

        int res = 0;
        for(int i = 0; i < n && !s.empty(); i++)
        {
//            if (s.find(num[i]) == s.end()) continue;  2014-12-10

            int upper = num[i], lower = num[i];

            while (s.find(upper+1) != s.end())
                s.erase(upper++);
            while (s.find(lower-1) != s.end())
                s.erase(lower--);

            res = max(res, upper-lower+1);
        }

        return res;
    }