118.杨辉三角 (Easy)*

class Solution {
public:
    vector<vector<int>> generate(int numRows) {
        vector<vector<int>> res(numRows);
        if(numRows == 0) {
            return res;
        }
        res[0].push_back(1);
        for(int i = 1; i < numRows; i++) {
            res[i].push_back(1);
            for(int j = 1; j < i; j++) {
                res[i].push_back(res[i - 1][j - 1] + res[i - 1][j]);
            }
            res[i].push_back(1);
        }
        return res;
    }
};