128. Longest Consecutive Sequence

Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

Your algorithm should run in O(n) complexity.

Example:

Input: [100, 4, 200, 1, 3, 2]
Output: 4
Explanation: The longest consecutive elements sequence is [1, 2, 3, 4]. Therefore its length is 4.

求最长连续序列的长度

C++:

class Solution {
private:
    unordered_map<int,int> cntMap ;
public:
    int longestConsecutive(vector<int>& nums) {
        int res = 0 ;
        for(int num : nums){
            cntMap[num] = 1 ;
        }
        for(int num : nums){
            forward(num) ;
        }
        for(int num : nums){
            res = max(res,cntMap[num]) ;
        }
        return res ;
    }
    
    int forward(int num){
        if (cntMap.count(num) == 0){
            return 0 ;
        }
        int cnt = cntMap[num] ;
        if (cnt > 1)
            return cnt ;
        
        cnt = forward(num+1) + 1 ;
        cntMap[num] = cnt ;
        return cnt ;
    }
};