LintCode_124 Longest Consecutive Sequence

Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

Clarification

Your algorithm should run in O(n) complexity.

Example

Given [100, 4, 200, 1, 3, 2],
The longest consecutive elements sequence is [1, 2, 3, 4]. Return its length: 4.

如果用排序算法，果断超时；

思路是这样的：对于每一个数，先左找连续的数，再右找连续的数， 然后把长度加起来。

查找是用的hash_map来实现。

但是此种方法会退化成O(n^2);

优化一下发现；对于先处理的4来说， 3，2，1都是同一个序列，所以不用重复计算，所以要把查找过的都删除。

class Solution {public:    /**     * @param nums: A list of integers     * @return an integer     */    int longestConsecutive(vector<int> &nums) {        // write you code here        unordered_set<int> set;        for (int i = 0; i < nums.size(); i++) {            set.insert(nums[i]);        }        int max_len = 0;        for (int i = 0; i < nums.size(); i++) {            int left_len = 0;            int right_len = 0;            int mid = nums[i];            while (set.find(mid - 1) != set.end()) {                left_len++;                mid--;                set.erase(mid);            }            mid = nums[i];            while (set.find(mid + 1) != set.end()) {                right_len++;                mid++;                set.erase(mid);            }            max_len = max(max_len, left_len + 1 + right_len);        }        return max_len;    }};