lintcode:Longest Consecutive Sequence

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本文介绍了一种寻找整数数组中最长连续元素序列的算法。通过使用哈希表存储数组元素，该算法能在O(n)的时间复杂度内找到最长连续序列的长度。示例中展示了一个具体的实现案例。

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

Example

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

Clarification

Your algorithm should run in O(n) complexity.

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class Solution {
public:
    /**
     * @param nums: A list of integers
     * @return an integer
     */
    int longestConsecutive(vector<int> &num) {
        // write you code here
        
        map<int, int> hashTable;
        
        for (int i=0; i<num.size(); i++)
        {
            if (hashTable.count(num[i]) == 0)
                hashTable[num[i]] = 1;
        }
        
        int maxLen = 0;
        for (int i=0; i<num.size(); i++)
        {
            if (hashTable.count(num[i]) != 0)
            {
                int curLen = 1;
                hashTable.erase(num[i]);
                
                int before = num[i]-1;
                while (hashTable.count(before) != 0)
                {
                    curLen++;
                    hashTable.erase(before);
                    
                    before--;
                }
                
                int after = num[i]+1;
                while (hashTable.count(after) != 0)
                {
                    curLen++;
                    hashTable.erase(after);
                    
                    after++;
                }
                
                maxLen = max(maxLen, curLen);
            }
        }
        
        return maxLen;
    }
};