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.

复杂度要求o(n)，用哈希或者构建辅助数组

把元素存入哈希

遍历数组，找每个元素能够引出的序列长度

class Solution {
public:
    int longestConsecutive(vector<int>& nums) {
        int numsSize = nums.size();
        unordered_map<int,bool> allNums;
        int res = 0;
        for(int i=0;i<numsSize;i++){
            allNums[nums[i]]=true;
        }
        for(int i=0;i<numsSize;i++){
            int cur = nums[i];
            if(!allNums[cur]){
                continue;
            }
            int cnt = 1;
            int pre = cur-1;
            while(allNums[pre]){
                cnt++;
                allNums[pre] = false;
                pre--;
            }
            int next = cur+1;
            while(allNums[next]){
                cnt++;
                allNums[next] = false;
                next++;
            }
            allNums[cur] = false;
            res = max(res,cnt);
        }
        return res;
    }
};