LeetCode —— Longest Consecutive Sequence
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链接:http://leetcode.com/onlinejudge#question_128
原题:
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:要O(N)时间找到最长连续串长度,完全没有思路,因为排序只是是O(nlogn)的;
参考别人的想法,http://blog.csdn.net/zxzxy1988/article/details/8589131
用hash做,才恍然大悟。C++ STL里新增了一个模板库unordered_map,就是基于hash表
实现的,查找、插入和删除都是O(1);
代码:
class Solution {public: int longestConsecutive(vector<int> &num) { // Start typing your C/C++ solution below // DO NOT write int main() function int max = 0; unordered_map<int, int> hashTable; for (int i=0; i<num.size(); i++) hashTable[num[i]] = num[i]; while (!hashTable.empty()) { int cur = 0; int target = (hashTable.begin())->second; cur += consecutiveLength(hashTable, target, -1); cur += consecutiveLength(hashTable, target+1, 1); if (cur > max) max = cur; } return max; } private: int consecutiveLength(unordered_map<int, int> &hashTable, int target, int step) { int count = 0; while (hashTable.find(target) != hashTable.end()) { count++; hashTable.erase(target); target += step; } return count; }};
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