just do it in a straightforward, each time generate a new line, first push all 1 vector into res and then update the elements from 1 to i-1.

```
class Solution {
public:
vector<vector<int>> generate(int numRows) {
vector<vector<int>> res;
for(auto i=0;i<numRows;++i)
{
res.push_back(vector<int>(i+1,1));
for(auto j=1; j<i; ++j) res[i][j] = res[i-1][j-1] + res[i-1][j];
}
return res;
}
};
```