[leetcode] Longest Consecutive Sequence

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本文介绍了一种算法，用于从无序整数数组中找出最长连续元素序列的长度，并确保算法的时间复杂度为O(n)。示例：对于数组[100,4,200,1,3,2]，最长连续序列是[1,2,3,4]，返回其长度4。

From : https://leetcode.com/problems/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.

class Solution {
public:
    int longestConsecutive(vector<int>& nums) {
        map<int, bool> has;
        int len = nums.size();
        for(int i=0; i<len; ++i) {
            has[nums[i]] = true;
        }
        
        int num = 0;
        for(int i=0; i<len; ++i) {
            int cur = 1;
            if(has[nums[i]]) {
                // left
                for(int k=nums[i]-1;; --k) {
                    if(has[k]) {
                        ++cur;
                        has[k] = false;
                    } else{
                        break;
                    }
                }
                // right
                for(int k=nums[i]+1;; ++k) {
                    if(has[k]) {
                        ++cur;
                        has[k] = false;
                    } else{
                        break;
                    }
                }
            }
            if(cur > num) {
                num = cur;
            }
        }
        return num;
        
    }
};