【LeetCode】Pascal's Triangle (杨辉三角)

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]
]

code :

class Solution {
public:
    vector<vector<int> > generate(int numRows) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        vector<vector<int> > res;
        if(numRows == 0)
            return res;
        for(int i = 1; i <= numRows; i++)
        {
            vector<int> onelevel;
            onelevel.clear();
            onelevel.push_back(1);
            for(int j = 1; j < i; j++)
            {
                onelevel.push_back(res[i-2][j-1] + (j < i-1 ? res[i-2][j] : 0));
            }
            res.push_back(onelevel);
        }
        return res;
    }
};