# My 32ms C++ solution with O(n) space

• ``````bool cmp(const pair<int,int> & a,const pair<int,int> & b)
{
return a.second < b.second;
}

class Solution {
public:
vector<int> findOrder(int num, vector<pair<int, int>>& pre) {
sort(pre.begin(),pre.end(),cmp);
vector<int> lim(num+1,-1);
vector<int> degree(num,0);
vector<int> ret;
for(auto x:pre)
degree[x.first]++;
for(int i = 1,cnt  = 0; i < pre.size(); i++)
if(pre[i-1].second!=pre[i].second)
lim[pre[i-1].second+1] = i;
if(pre.size())
lim[pre.back().second+1] = pre.size();
for(int i = 0,llast = 0; i <= num; i++)
if(lim[i] < 0)
lim[i] = llast;
else
llast = lim[i];
queue<int> que;
for(int i = 0; i < num; i++)
if(!degree[i])
que.push(i);
for(int j = 0; j < num; j++)
{
if(que.empty())
return vector<int>();
const int t = que.front();que.pop();
ret.push_back(t);
for(int i = lim[t]; i < lim[t+1];  i++)
{
degree[pre[i].first]--;
if(!degree[pre[i].first])
que.push(pre[i].first);
}
}
return ret;
}
};``````

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