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> > res(numRows);
        for(int i = 0; i < numRows; i++) {
            res[i].push_back(1);
            for(int j = 1; j < i; j++) {
                res[i].push_back(res[i-1][j-1] + res[i-1][j]);
            }
            if(i >= 1) res[i].push_back(1);
        }
        return res;
    }
};