[LeetCode]128.Longest Consecutive Sequence
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【题目】
Longest Consecutive Sequence
Total Accepted: 4743 Total Submissions: 17989My SubmissionsGiven 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.
【题意】
给出一个没有排序的整数数组,找出最长的连续元素序列的长度。
例如,
给定[100,4,200,1,3,2],
最长的连续元素序列是[1,2,3,4]。返回它的长度:4。
你的算法应该运行在O(n)的复杂度。
【分析】
如果允许 O(n log n) 的复杂度,那么可以先排序,可是本题要求 O(n)。由于序列里的元素是无序的,又要求 O(n),首先要想到用哈希表。
用一个哈希表visited[]记录每个元素是否使用,对每个元素,以该元素为中心,往左右扩张,直到不连续为止,记录下最长的长度。
在[100,4,200,1,3,2,101,99,97,98,102]的例子中:
(1)第一步:以100为中心
向右扩张:101,102
向左扩张:99,98,97
(2)第二步:以4为中心
向右扩张:无
向左扩张:3,2,1
(3)第三步:以200为中心
向右扩张:无
向左扩张:无
(4)1,3,2,101........以被扩张,无需以他们为中心进行扩张【代码】
/********************************** 日期:2014-01-17* 作者:SJF0115* 题号: Longest Consecutive Sequence* 来源:http://oj.leetcode.com/problems/longest-consecutive-sequence/* 结果:AC* 来源:LeetCode* 总结:**********************************/#include <iostream>#include <stdio.h>#include <vector>#include <tr1/unordered_map>using namespace std;using namespace tr1;class Solution {public: int longestConsecutive(vector<int> &num) { int i,j; int CurLength = 0; int MaxLength = 0; int Len = num.size(); // Map中的元素是自动按key升序排序 unordered_map<int,bool> used; //初始化 for(i = 0;i < Len;i++){ used[num[i]] = false; } for(i = 0;i < Len;i++){ //如果以扩张,跳过 if(used[num[i]]){ continue; } else{ //以num[i]为中心左右扩张,直到不连续为止 used[num[i]] = true; CurLength = 1; //向右扩张 for(j = num[i] + 1;used.find(j) != used.end();j++){ CurLength ++; used[j] = true; } //向左扩张 for(j = num[i] - 1;used.find(j) != used.end();j--){ CurLength ++; used[j] = true; } //更新最大长度 if(CurLength > MaxLength){ MaxLength = CurLength; } }//if }//for return MaxLength; }};int main() { int result; Solution solution; vector<int> vec; vec.push_back(100); vec.push_back(4); vec.push_back(200); vec.push_back(1); vec.push_back(3); vec.push_back(2); vec.push_back(101); vec.push_back(99); vec.push_back(97); vec.push_back(98); vec.push_back(102); result = solution.longestConsecutive(vec); printf("Result:%d\n",result); return 0;}
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