[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.

O(n) 的复杂度。这道题肯定要遍历每个数，那么对每个数的操作时间就是 O(1)，考虑用 hashmap。hashmap 的 find 函数复杂度在大多数时间为 O(1)。此处的 hashmap 用的是 STL 中的 unordered_map。

class Solution {
public:
    int longestConsecutive(vector<int> &num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        // 空间换时间
        // Map 的 find 复杂度为 Log
        // Hashmap 的 find 复杂度为 O(1)
        
        if(num.size()==0) return 0;
        
        unordered_map <int,int> indexMap; // hashmap
        
        int length = 0;
        int maxLength = 0;
        
        for(int i=0;i<num.size();i++)
        {
            indexMap[num[i]] = i;
        }
        
        vector<bool> visited(num.size(),false);
        
        for(int i=0;i<num.size();i++)
        {
            if(visited[i]==true)
            {
                continue;
            }
            
            visited[i] = 1;
            int index = num[i]+1;
            length = 1;
            while(indexMap.find(index)!=indexMap.end()) // 复杂度 O(1)
            {
                visited[indexMap[index]] = true;
                length++;
                index++;                
            }
            index = num[i]-1;
            while(indexMap.find(index)!=indexMap.end())
            {
                visited[indexMap[index]] = true;
                length++;
                index--;                
            }
            maxLength = max(maxLength,length);
        }
        
        return maxLength;
        
    }
};