Pascal's Triangle [LeetCode]

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> > vec;         if(numRows == 0) return vec;         vector<int> line;         line.push_back(1);         vec.push_back(line);                for(int i=2;i<=numRows; ++i){            vector<int> rows;            rows.clear();            rows.push_back(1);            for(int j=0;j<i-2;++j){                rows.push_back(vec[i-2][j]+vec[i-2][j+1]);            }            rows.push_back(1);            vec.push_back(rows);        }        return vec;    }};