LC128. Longest Consecutive Sequence
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.
这题，不知道Union find怎么做呀？不过discuss的Hashmap做法是很好的，对于每一个元素，存入连续的数目，不断更新即可。
public class Solution {
// hashmap trad off of time and space
public int longestConsecutive(int[] nums) {
if(nums == null) return 0;
int res = 0;
HashMap<Integer, Integer> map = new HashMap<>();
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);
res = Math.max(res, sum);
map.put(n-left, sum);
map.put(n+right, sum);
}else{
continue;
}
}
return res;
}
}