[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;
    }
};