[LeetCode]Longest Consecutive Sequence

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.

思考：空间换时间，查找O(1)用unordered_set。

class Solution {

public:

    int longestConsecutive(vector<int> &num) {

        // Start typing your C/C++ solution below

        // DO NOT write int main() function

        unordered_set<int> existTable;

        for(int i = 0; i < num.size(); ++i) existTable.insert(num[i]);

        unordered_set<int> visitedTable;

        int mmax = INT_MIN;

        for(int i = 0; i < num.size(); ++i)

        {

            int curNum = num[i];

            if(visitedTable.find(curNum) != visitedTable.end()) continue;

            int cnt = 1;

            //search left

            int left = curNum;

            while(existTable.find(--left) != existTable.end()) cnt++, visitedTable.insert(left);

            int right = curNum;

            while(existTable.find(++right) != existTable.end()) cnt++, visitedTable.insert(right);

            mmax = max(cnt, mmax);

        }

        return mmax;

    }

};