leetcode刷题系列C++-Longest Consecutive 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.

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/*
使用哈希表 找出最长的连续的序列
*/
class Solution {
public:
    int longestConsecutive(vector& nums) {
        unordered_map used; //声明哈希map
        for(auto i : nums)
            used[i] = false;//里面的每一个数字都作为一个key  同时value为false
        int longest = 0;
        for(auto i : nums)
        {
            if(used[i])
                continue;
            int length = 1;
            used[i] = true;
            for(int j = i + 1;used.find(j) != used.end();++j)
            {
                used[j] = true;
                ++length;
            }
            for(int j = i - 1; used.find(j) != used.end(); --j)
            {
                used[j] = true;
                ++length;
            }
            longest = max(longest,length);
        }
        return longest;
    }
};

编程步骤：

将数组里的每一个数字作为key存在哈希map中，value值用bool表示，然后从增加的方向和减少的方向分别找当前元素的最长距离，一旦该元素使用过 就将该元素的bool值设置为true 一次遍历就可以得到结果