leetcode 128 —— 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.

思路：使用unordered_set 记录所有点,然后往往两边搜索class Solution {
public:
int longestConsecutive(vector<int>& nums) {
unordered_set<int> iSet;
int maxLen = 0;
for (int i = 0; i < nums.size(); i++) //将所有情况存入set
iSet.insert(nums[i]);
for (int i = 0; i < nums.size(); i++){
int cnt = 1;
if (iSet.count(nums[i]) == 0)
continue;
int j = 1; //往两边搜索，并把搜到的从原set里删除
while (iSet.count(nums[i] - j)>0){
iSet.erase(nums[i] - j++);
cnt++;
}
j = 1;
while (iSet.count(nums[i] + j) > 0){
iSet.erase(nums[i] + j++);
cnt++;
}
maxLen = maxLen > cnt ? maxLen : cnt;
}
return maxLen;
}
};