LeetCode: Longest Consecutive Sequence

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.

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

算法：其实这道题最重要的是利用map中可以按关键字有序遍历的特性。首先，将数组中所有元素存储在map里，这样相当与可以在线性时间内对数组排序，其实就是用空间换时间的方法。代码：

class Solution {
public:
    int longestConsecutive(vector<int> &num) {
        if(num.empty())    return 0;
        map<int, int> hash_table;
        for(int i = 0; i < num.size(); ++i){
            hash_table[num[i]] = num[i];
        }
        map<int,int>::iterator it = hash_table.begin();
        int max_length = 1;
        int pre_ele = it->first;
        int length = 1;
        ++it;
        for(; it != hash_table.end(); ++it){
            if(it->first == pre_ele + 1){
                ++length;
            }else{
                if(length > max_length){
                    max_length = length;
                }
                length = 1;
            }
            pre_ele = it->first;
        }
        if(length > max_length)
            max_length = length;
        return max_length;
    }
};