Pascal's Triangle

class Solution {
public:
vector<vector<int>> generate(int numRows) {
vector<vector<int>> vv;
vector<int> v_arr[numRows+1];

if( numRows <= 0) return vv;
v_arr[1].push_back(1);
vv.push_back(v_arr[1]);
if(numRows == 1) return vv;
for(int row=2;row<=numRows;row++){
v_arr[row].push_back(1);       // 1 at begin
int pre_len = v_arr[row-1].size();
for(int i=0;i<pre_len-1;i++){
v_arr[row].push_back(v_arr[row-1][i]+v_arr[row-1][i+1]);
}
v_arr[row].push_back(1);     //1 at end
vv.push_back(v_arr[row]);
}

return vv;

}
};