[LeetCode]128. Longest Consecutive Sequence

128. 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.

分析

由於需要在先行複雜度內完成，很自然想到需要使用哈希表，利用哈希表來記錄元素是否被訪問。遍歷數組時，選定一個數後，向該數的兩邊尋找連續的元素，直到沒有連續元素爲止。

源碼

class Solution {
public:
    int longestConsecutive(vector<int>& nums) {
        map<int,bool> mapper; // 使用哈希表可以O(1)時間查找該數是否被訪問過
        for(int i = 0; i < nums.size(); ++i) {
            mapper[nums[i]] = false;
        }

        int longest = 0;

        for(int i = 0; i < nums.size(); ++i) {
            if(mapper[nums[i]]) continue;//訪問過則不繼續檢查

            int innerLongest = 1;
            //向兩邊的連續數做檢查
            for(int right = nums[i] + 1; mapper.find(right) != mapper.end(); ++right) {
                if(mapper[right] == false) {
                    mapper[right] = true;
                    innerLongest += 1;
                }
            }

            for(int left = nums[i] - 1; mapper.find(left) != mapper.end(); --left) {
                if(mapper[left] == false) {
                    mapper[left] = true;
                    innerLongest += 1;
                }
            }

            longest = longest > innerLongest ? longest : innerLongest;
        }

        return longest;
    }
};