[leetcode] 118 Pascal Triangle c++

/*
http://leetcode.com/onlinejudge#question_118

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) {
        typedef vector<int> row_t;
        typedef vector<row_t> tri_t;

        tri_t tri;

        for(size_t row_no = 0; row_no < numRows; ++row_no)
        {
            const size_t row_size = row_no + 1;

            tri.push_back(row_t(row_size));
            tri[row_no][0] = tri[row_no][row_no] = 1;

            for(size_t pos = 1; pos < row_no; ++pos)
            {
                tri[row_no][pos] =
                    tri[row_no-1][pos-1] + tri[row_no-1][pos];
            }

        }

        return tri;
    }
};