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 > generate(int numRows) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<vector<int> > ret;
        if (!numRows) return ret;
        vector<int> firstRow;
        firstRow.push_back(1);
        ret.push_back(firstRow);
        
        for (int i = 1; i < numRows; ++i) {
            vector<int> lastRow = ret[i - 1];
            vector<int> row;
            for (int j = 0; j <= i; ++j) {
                if (j != 0 && j != i) {
                    row.push_back(lastRow[j] + lastRow[j - 1]);
                }
                else if (j == 0) {
                    row.push_back(lastRow[j]);
                }
                else {
                    row.push_back(lastRow[j - 1]);
                }
            }
            ret.push_back(row);
        }
        
        return ret;
    }
};