Pascal's Triangle

class Solution {
public:
    vector<vector<int> > generate(int numRows) {
        int i,j;
        if(numRows==0)
        {
            return vector<vector<int> >();
        }
        vector<vector<int> > vec1; 
        vector<int> vec2;
        vec2.push_back(1);
        vec1.push_back(vec2);
        for(i=1;i<numRows;i++)
        {
            vec2.clear();
            vec2.push_back(1);
            for(j=1;j<i;j++)
            {
                vec2.push_back(vec1[i-1][j]+vec1[i-1][j-1]);
            }
            vec2.push_back(1);
            vec1.push_back(vec2);
        }
        return vec1;
    }
};