标签：style class blog code tar color
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.
方法一
先排序，然后找就好了，但是要求是O(n),排序明显是个O(n*logn)的算法.
方法二
只是找连续的嘛,我们可以把所有的数字都存入hash表,然后随意从某个数字开始找他的前面和后面那个是否存在.
然后得到一个最大的长度.当然找过的就可以删掉了...你想,一个连续的序列,你从中间任意位置开始往两边找不都一样么.
所以只要找过就可以删掉了.
1 class Solution { 2 public: 3 set<int> flag; 4 int findBound(int n , bool asc){ 5 int ans = 0; 6 set<int>::iterator iter; 7 while((iter = flag.find(n)) != flag.end()){ 8 flag.erase(iter); 9 ans ++; 10 if(asc) 11 n-- ; 12 else 13 n++; 14 } 15 return ans; 16 } 17 int longestConsecutive(vector<int> &num) { 18 // Start typing your C/C++ solution below 19 // DO NOT write int main() function 20 21 int ans = 0; 22 flag.clear(); 23 for(int i = 0 ; i < num.size() ; i++) 24 flag.insert(num[i]); 25 for(int i = 0 ; i < num.size(); i++){ 26 ans = max(findBound(num[i],true) + findBound(num[i]+1,false) , ans); 27 } 28 return ans; 29 } 30 };
下面用hash代替了set，思想一样，看上去没有上面的简洁
1 class Solution { 2 public: 3 int longestConsecutive(vector<int> &num) { 4 // Start typing your C/C++ solution below 5 // DO NOT write int main() function 6 map<int, int> hmap; 7 hmap.clear(); 8 int n = num.size(); 9 for(int i=0; i<n; i++){ 10 hmap.insert(pair<int, int>(num[i], i)); 11 } 12 int ans=0, cnt=0; 13 map<int, int>::iterator it; 14 for(int i=0; i<num.size(); i++){ 15 int cur = num[i]; 16 it = hmap.find(num[i]); 17 cnt = 1; 18 if(it!=hmap.end()){ 19 map<int, int>::iterator iter; 20 while(1){ 21 iter = hmap.find(++cur); 22 if(iter==hmap.end()) 23 break; 24 cnt++; 25 hmap.erase(iter); 26 } 27 cur=num[i]; 28 while(1){ 29 iter = hmap.find(--cur); 30 if(iter==hmap.end()) 31 break; 32 cnt++; 33 hmap.erase(iter); 34 } 35 if(ans<cnt) 36 ans = cnt; 37 38 } 39 40 } 41 return ans; 42 } 43 };
[LeetCode] Longest Consecutive Sequence,布布扣,bubuko.com
[LeetCode] Longest Consecutive Sequence
标签：style class blog code tar color
原文地址：http://www.cnblogs.com/diegodu/p/3791119.html
