leetcode-Longest Consecutive Sequence(2014.3.31)
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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.
肯定 不能排序,排序的时间复杂度至少是O(nlogn).涉及到时间的限制,大多要使用哈希。本题用map有点 多余,set足以 。
class Solution {
public:
int longestConsecutive(vector<int> &num) {
int iCount=0;
set<int> hash_map;
set<int>::iterator it;
for(int i=0;i<num.size();i++){
if(hash_map.find(num[i])==hash_map.end()){
hash_map.insert(num[i]);
}
}
for(int i=0;i<num.size();i++){
int iCount1=1;
int p=num[i]-1;
while(hash_map.find(p)!=hash_map.end()){
hash_map.erase(p);
iCount1++;
p--;
}
p=num[i]+1;
while(hash_map.find(p)!=hash_map.end()){
hash_map.erase(p);
iCount1++;
p++;
}
if(iCount1>iCount){
iCount=iCount1;
}
}
return iCount;
}
};
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