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