Array - 128. Longest Consecutive Sequence

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.

思路：

题目提示必须跑在O(n)，可以使用set或者map来做，就是online和offline，offline非常简单直接，就是利用set的O(1)操作来解。

代码：

java：

class Solution {

    // online 
    public int longestConsecutive(int[] nums) {
        if( nums == null ||nums.length == 0) return 0;
        
        int res = 0;
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        for (int n : nums) {
            if(!map.containsKey(n)) {
                int left = map.containsKey(n-1) ? map.get(n-1) : 0;
                int right = map.containsKey(n+1) ? map.get(n+1) : 0;
                
                int sum = left + right + 1;
                
                res = Math.max(res, sum);
                map.put(n, sum);
                
                map.put(n - left, sum);
                map.put(n + right, sum);
            }
        }
        
        return res;
    }
    
    // offline
   /* public int longestConsecutive(int[] nums) {
        if (nums == null || nums.length == 0) return 0;
        
        Set set = new HashSet();
        for (int num : nums) set.add(num);
        
        int ans =0;
        for (int n : nums){
            if (!set.contains(n-1)) {
                int len = 0;
                while (set.contains(n++)) ++len;
                ans = Math.max(ans, len);
            }
        }
        
        return ans;
    }*/
}