leetcode Question 64: Pascal's Triangle I

Pascal's Triangle I:

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]
]

Analysis:
In each row, the first and last element are 1. And the other element is the sum of the two elements in the previous row. e.g.

   1 3 3 1   Previous row
1 1+3 3+3 3+1 1   Next row

   1 4 6 4 1 Previous row
1 1+4 4+6 6+4 4+1 1 Next row

So the idea is simple:
(1) Add 1 to current row.
(2) Get the previous line. Sum every two elements and add to current row.
(3) Add 1 to current row.
(4) Go fill the next row


Code ( updated 2013.08 ):

class Solution {
public:
    vector<vector<int> > generate(int numRows) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<vector<int> > res;
        if (numRows==0){return res;}
        vector<int> r;
        r.push_back(1);
        res.push_back(r);
        if (numRows==1){return res;}
        r.push_back(1);
        res.push_back(r);
        if (numRows==2){return res;}
        
        for (int i=2;i<numRows;i++){
            vector<int> c(i+1,1);
            for (int j=1;j<i;j++){
                c[j]= res[i-1][j]+res[i-1][j-1];
            }
            res.push_back(c);
        }
        return res;
    }
};