leetcode刷题系列C++-Longest Consecutive 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.

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/*
使用哈希表 找出最长的连续的序列
*/
class Solution {
public:
int longestConsecutive(vector<int>& nums) {
unordered_map<int,bool> used; //声明哈希map
for(auto i : nums)
used[i] = false;//里面的每一个数字都作为一个key  同时value为false
int longest = 0;
for(auto i : nums)
{
if(used[i])
continue;
int length = 1;
used[i] = true;
for(int j = i + 1;used.find(j) != used.end();++j)
{
used[j] = true;
++length;
}
for(int j = i - 1; used.find(j) != used.end(); --j)
{
used[j] = true;
++length;
}
longest = max(longest,length);
}
return longest;
}
};

编程步骤：

将数组里的每一个数字作为key存在哈希map中，value值用bool表示，然后从增加的方向和减少的方向分别找当前元素的最长距离，一旦该元素使用过 就将该元素的bool值设置为true 一次遍历就可以得到结果