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LeetCode 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.
解题思路:首先要注意到时间复杂度是O(n),那么问题来了,只能扫描数组一遍,显然来不及排序了(放弃排序),注意到只要返回一个整数,考虑使用map,对于遍历的数组中的数用hash存储,并且在端点保存连续的大小,在遍历下一个数时,看他的左右是否存在,若存在则更新端点的长度,否则,插入该数,如果该数在hash中,跳过。
class Solution {public: int longestConsecutive(vector<int> &num) { int res = 0; unordered_map<int,int> hashMap; for(auto x: num) { if(!hashMap[x])//if not exit { hashMap[x] = 1 + hashMap[x-1] + hashMap[x+1]; if(hashMap[x+1])hashMap[hashMap[x+1]+x] = hashMap[x];//update the right border if(hashMap[x-1])hashMap[x-hashMap[x-1]] = hashMap[x];//update the left border } res = max(res,hashMap[x]); } return res; }};
LeetCode Longest Consecutive Sequence
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