【Leetcode】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.

数据元素无序但要求时间复杂度O(n)应联想到hash.

stl中unordered_map(c++ 11) 和 hash_map 速度比map(红黑树实现)快。新的标准推荐用unordered_map代替hash_map。

class Solution {
public:
    int longestConsecutive(vector<int> &num) {
        unordered_map<int, bool> table;
        for (auto i : num) {
            table[i] = false;
        }
        int ret = 0;
        for (auto i : num) {
            if (table[i]) continue;
            table[i] = true;
            int len = 1;
            for (int j = i + 1; table.find(j) != table.end(); ++j) {
                ++len;
                table[j] = true;
            }
            for (int j = i - 1; table.find(j) != table.end(); --j) {
                ++len;
                table[j] = true;
            }
            if (len > ret) ret = len;
        }
        return ret;
    }
};