128. Longest Consecutive Sequence

Posted 2020-09-07 为了更优秀的你，加油！

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.

class Solution {
public:
    /*对于当前的n每次查看n-1和n+1是否已经在map中，left记录n左边的连续长度，right记录n右边的连续长度
    当前长度为left+right+1
    更新当前能扩展到的位置的值
    */
    int longestConsecutive(vector<int>& nums) {
        unordered_map<int,int>map;
        int left,right,sum,res=0;
        for(int n: nums){
            if(!map[n]){
                left=map[n-1]?map[n-1]:0;
                right=map[n+1]?map[n+1]:0;
                sum=left+right+1;
                map[n]=sum;
                res=max(res,sum);
                map[n-left]=sum;
                map[n+right]=sum;
            }
        }
        return res;
    }
};