Pascal's Triangle

Given numRows, generate the first numRows of Pascal's triangle.

For example, given numRows = 5,
Return

[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

class Solution {
public:
    vector<vector<int> > generate(int numRows) {
        vector<vector<int> > result;
        if (numRows < 1)
        {
            return result;
        }

        vector<int> buf(1,1);
        result.push_back(buf);
        for (int i = 1; i < numRows; i++)
        {
            vector<int> temp;
            for (int j = 0; j < result[i-1].size()+1; j++)
            {
                int size = temp.size();
                if (size == 0 || size == result[i-1].size())
                {
                    temp.push_back(result[i-1][0]);
                }
                else
                {
                    temp.push_back(result[i-1][j-1] + result[i-1][j]);
                }
            }
            result.push_back(temp);
        }

        return result;
    }
};