Longest Consecutive Sequence

最新推荐文章于 2024-08-13 15:34:52 发布

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本文探讨了如何在未排序的整数数组中找到最长连续整数序列的长度，使用了Map来实现O(n)复杂度的算法。

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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.

Your algorithm should run in O(n) complexity.

这种思想主要是运用了Map的自动排序的特性

class Solution {
public:
    int longestConsecutive(vector<int> &num) {
        
        std::map<int, int> countMap;
        
        int len = num.size();
        
        for (int i=0; i<len; i++)
        {
            if (countMap[num[i]] == 0)
            {
                countMap[num[i]] = 1;
            }
        }
        
        int curVal = 0;
        int maxVal = 0;
        std::map<int, int>::iterator itr = countMap.begin();
        int pre = itr->first;
        curVal++;
        maxVal++;
        ++itr;

        while (itr != countMap.end())
        {
            if (itr->first == pre+1)
            {
                curVal++;
                if (curVal > maxVal)
                {
                    maxVal = curVal;
                }
            }
            else
            {
                curVal = 1;
            }
            
            pre = itr->first;
            ++itr;
        }
        
        return maxVal;
        
    }
};

还有一种思想参考：http://blog.youkuaiyun.com/aivin24/article/details/23873943

class Solution {
public:
    int longestConsecutive(vector<int> &num) {
        unordered_map<int,int> f;
		int max_v(1);
		for(auto val: num)
			f[val] = 1;
		for(auto val: num){
			int left = val - 1, right = val + 1;
			while (f[left]){
				f[left] = 0;
				left--;
			}
			while(f[right]){
				f[right] = 0;
				right++;
			}
			max_v = max(max_v,right-left-1);
		}
		return max_v;
    }
};