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;
          
    }
};