【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;
}
};