【leetcode】Longest Consecutive Sequence

鏈接：https://oj.leetcode.com/problems/longest-consecutive-sequence/

描述：

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.

方法：

基本想法是排序然後遍歷，但是要求O(N)複雜度，考慮空間換時間，兩遍遍歷。

代碼如下：

 int longestConsecutive(vector<int> &num) {
    	int len = num.size();
    	if(len <= 0) return 0;
    	map<int, int> hash;
    	for(int i=0; i < len; ++i)
    		hash[num[i]] = 1;
    	int count  = 0; 
    	int max = 0;
    
    	for(int i=0; i < len; ++i)
    	{
    		int curNum = num[i];
    		if(hash[curNum] == 1)
    		{	
    			count = 1;
    			int next = curNum -1;
    			while( hash[next] == 1)
    			{
    				count++;
    				hash[next--] = 2;
    			}
    			next = curNum +1;
    			while(hash[next] == 1)
    			{
    				count++;
    				hash[next++] = 2;
    			}
    			if( max < count )
    			    max = count;
    		}
    	}
    	return max;
    }