Array Pascal's Triangle

思路：

下一层根据上一层计算出来，不断加入每层的vector。

class Solution {
public:
    vector<vector<int>> generate(int numRows) {
        vector<vector<int>> ans;
        if(numRows == 0) return ans;
        vector<int> row_one = {1};
        ans.push_back(row_one);
        if(numRows == 1) return ans;
        for(int i = 2; i <= numRows; ++i) {
            vector<int> row;
            row.push_back(1);
            for(int j = 1; j <= i-2; ++j) {
                row.push_back(ans[i-2][j-1] + ans[i-2][j]);
            }
            row.push_back(1);
            ans.push_back(row);
        }
        return ans;
    }
};