Longest Consecutive Sequence leetcode java

题目：

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.

 

题解：

这道题利用HashSet的唯一性解决，能使时间复杂度达到O(n)。首先先把所有num值放入HashSet，然后遍历整个数组，如果HashSet中存在该值，就先向下找到边界，找的同时把找到的值一个一个从set中删去，然后再向上找边界，同样要把找到的值都从set中删掉。所以每个元素最多会被遍历两边，时间复杂度为O(n)。

代码如下：

public int longestConsecutive(int[] num) {          if(num == null||num.length == 0)            return 0;                HashSet<Integer> hs = new HashSet<Integer>();                  for (int i = 0 ;i<num.length; i++)               hs.add(num[i]);                   int max = 0;          for(int i=0; i<num.length; i++){              if(hs.contains(num[i])){                int count = 1;                  hs.remove(num[i]);                                int low = num[i] - 1;                 while(hs.contains(low)){                      hs.remove(low);                      low--;                      count++;                  }                                int high = num[i] + 1;                  while(hs.contains(high)){                      hs.remove(high);                      high++;                      count++;                  }                  max = Math.max(max, count);              }          }          return max;      }