LeetCode:Longest Consecutive Sequence

Longest Consecutive Sequence

Total Accepted: 67348 Total Submissions: 206214 Difficulty: Hard

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.

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思路：

使用一个Map保存连续序列两个边缘的值，这个值表示这个序列最大宽度。

如有nums = [1,2,3,4,5]，则map.get(1) = map.get(5) = 5;

设有nums = [1,2,5,6,3,4].

    1 2 3 4 5 6

1: 1

2: 2 2

5: 2 2       1

6: 2 2       2 2

3: 3 2 3    2 2

4: 6 2 3 6 2 6

java code:

public class Solution {    public int longestConsecutive(int[] nums) {                int ans = 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;                map.put(n, sum);                map.put(n-left, sum);                map.put(n+right, sum);                                ans = Math.max(ans, sum);            }        }        return ans;    }}