考察了一些二维vector的使用方法，我和题解写的差不多，题解里用了一个resize函数调整vector大小。

#include<iostream>
#include<vector>
using namespace std;
class Solution {
public:
    vector<vector<int>> generate(int numRows) {
        vector<vector<int>>ans(numRows);//行数确定，列数不确定  
        ans[0].push_back(1);
        vector<int>a;
        for (int i = 1; i <numRows; i++) {
            for (int j = 0; j <=i; j++) {
                if (j == 0 || j == i)a.push_back(1);
                else a.push_back(ans[i - 1][j - 1] + ans[i - 1][j]);
            }
            ans[i] = a;
            a.clear();
        }
        return ans;
    }
};
//题解做法
class Solution1 {
public:
    vector<vector<int>> generate(int numRows) {
        vector<vector<int>> ret(numRows);
        for (int i = 0; i < numRows; ++i) {
            ret[i].resize(i + 1);//resize函数可以调节vector的大小
            ret[i][0] = ret[i][i] = 1;
            for (int j = 1; j < i; ++j) {
                ret[i][j] = ret[i - 1][j] + ret[i - 1][j - 1];
            }
        }
        return ret;
    }
};
int main() {
    Solution test;
    vector<vector<int>>ans = test.generate(5);
    for (int i = 0; i < ans.size(); i++) {
        for (int j = 0; j < ans[i].size(); j++) {
            cout << ans[i][j];
        }
        cout << endl;
    }
  
}