Pascal's Triangle

class Solution {
public:
    vector<vector<int>> generate(int numRows) {
        vector<vector<int>>result;
        if(numRows<=0)
        return result;
        int n = numRows;
      for(int i= 0;i<n;i++)
      {
           vector<int>temp;
           for(int j=0;j<=i;j++)
           {if(j==0||j==i)//技巧就是这里的第一个和最后都为1，用该语句
                temp.push_back(1);
           else
                temp.push_back(result[i-1][j-1]+result[i-1][j]);
           }
          result.push_back(temp);
      }
      return result; 
    }
};