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：
代码语言：javascript复制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;
}*/
}
