给定一整数，求Pascal三角形。

vector
     
      
       > generate(int numRows) {    vector
       
        
          > ret;        for (int i = 0; i < numRows; ++i) {        vector
         
           ins;        for (int j = 0; j <= i; ++j) {            if (j == 0 || j == i)                ins.push_back(1);            else                ins.push_back(ret[i - 1][j - 1] + ret[i - 1][j]);        }        ret.push_back(move(ins));    }    return move(ret);}