Pascal's Triangle II

class Solution {
public:
vector<int> getRow(int rowIndex) {
vector<int> v;
if( rowIndex < 0) return v;

v.push_back(1);
if(rowIndex == 0) return v;

v.push_back(1);
if(rowIndex == 1) return v;

vector<int> v1;

for(int row=2;row<=rowIndex;row++){

v1.push_back(1);
int pre_len = v.size();
for(int i=0;i<pre_len-1;i++){
v1.push_back(v[i]+v[i+1]);
}
v1.push_back(1);
v=v1;      //vector 拷贝好方便啊
v1.clear();
}
return v;
}
};