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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 (M) Binary Tree Longest Consecutive Sequence

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;
}
}