LEETCODE: Longest Consecutive Sequence

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最新推荐文章于 2025-07-05 01:17:14 发布

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文章标签： LEETCODE 算法 hash

本文链接：https://blog.youkuaiyun.com/anyicheng2015/article/details/42362425

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本文介绍了一种使用哈希表解决寻找未排序整数数组中最长连续整数序列的方法，算法复杂度为O(n)，并详细解释了实现步骤。

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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.

用map来记录，实际上也就是用map里的hash方法来解决这个问题。

class Solution {
public:
    int longestConsecutive(vector<int> &num) {
        map<int, int> themap;
        for(int ii = 0; ii < num.size(); ii ++) {
            themap[num[ii]] = 1;
        }
        int max = 0;
        for(int ii = 0; ii < num.size(); ii ++) {
            int current = num[ii];
            if(themap[current] == 0) continue;
            themap[current] = 0;
            int len = 1;
            // Search to left.
            int toleft = current - 1;
            while(themap.find(toleft) != themap.end()) {
                len ++;
                themap[toleft] = 0;
                toleft --;
            }
            // Search to right.
            int toright = current + 1;
            while(themap.find(toright) != themap.end()) {
                len ++;
                themap[toright] = 0;
                toright ++;
            }
            if(len > max) {
                max = len;
            }
        }
        return max;
    }
};