LeetCode ： Pascal's Triangle

常规，一层一层来

vector<vector<int>> generate(int numRows)
{
    vector<vector<int>> res;
    if(numRows < 1)
        return res;
    vector<int> first(1, 1);
    res.push_back(first);

    for(int i = 1; i < numRows; i++)
    {
        vector<int> v(1, 1);
        for(int j = 1; j < i; j ++)
        {
            v.push_back(first[j - 1] + first[j]);
        }
        v.push_back(1);     
        res.push_back(v);
        first = v;
    }

    return res;

}