写代码，测试代码性能的时候往往需要测量代码执行时间，这时候就需要计时。现总结自己常用的计时方式如下：
c方式的计时
#include <time.h>
double dur;
clock_t start, end;
start = clock();
// operations
end = clock();
dur = (double)(end - start);
printf("Use Time:%f\n", (dur*1000 / CLOCKS_PER_SEC)); // ms
c++ 11 chrono方式的计时
#include <chrono>
auto start = std::chrono::steady_clock::now();
// operations
auto end = std::chrono::steady_clock::now();
std::chrono::duration<double> elapsed_seconds = end-start;
std::cout << "It took " << elapsed_seconds.count() << " seconds.";
