LeetCode —— Longest Consecutive Sequence

链接：http://leetcode.com/onlinejudge#question_128

原题：

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.

思路：要O(N)时间找到最长连续串长度，完全没有思路，因为排序只是是O(nlogn)的；

参考别人的想法，http://blog.csdn.net/zxzxy1988/article/details/8589131

用hash做，才恍然大悟。C++ STL里新增了一个模板库unordered_map，就是基于hash表

实现的，查找、插入和删除都是O（1）；

代码：

class Solution {
public:
    int longestConsecutive(vector<int> &num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int max = 0;
        unordered_map<int, int> hashTable;
        for (int i=0; i<num.size(); i++)
            hashTable[num[i]] = num[i];
        while (!hashTable.empty()) {
            int cur = 0;
            int target = (hashTable.begin())->second;
            cur += consecutiveLength(hashTable, target, -1);
            cur += consecutiveLength(hashTable, target+1, 1);
            if (cur > max)
                max = cur;
        }
        
        return max;
    }
    
private:
    int consecutiveLength(unordered_map<int, int> &hashTable, int target, int step) {
        int count = 0;
        while (hashTable.find(target) != hashTable.end()) {
            count++;
            hashTable.erase(target);
            target += step;
        }
        return count;
    }
};