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;    }};