请你回答一下STL里resize和reserve的区别？

resize()：改变当前容器内含有元素的数量(size())，eg: vectorv; v.resize(len);v的size变为len,如果原来v的size小于len，那么容器新增（len-size）个元素，元素的值为默认为0.当v.push_back(3);之后，则是3是放在了v的末尾，即下标为len，此时容器是size为len+1；

#include <iostream>#include <vector>using namespace std;int main() {    vector<int> a;    a.reserve(100);    a.resize(50);    cout<<a.size()<<"  "<<a.capacity()<<endl;        //50  100    a.resize(150);    cout<<a.size()<<"  "<<a.capacity()<<endl;        //150  200    a.reserve(50);    cout<<a.size()<<"  "<<a.capacity()<<endl;        //150  200    a.resize(50);    cout<<a.size()<<"  "<<a.capacity()<<endl;        //50  200    }