File content as of revision 0:9a525a0d1d3f:

#include "mbed.h"
#include "N5110.h"

N5110 lcd(PTC9,PTC0,PTC7,PTD2,PTD1,PTC11); 

InterruptIn buttonL(PTB18);
InterruptIn buttonR(PTB3);

// g_ prefix makes it easier to distinguish it as global
volatile int g_buttonL_flag = 0;
volatile int g_buttonR_flag = 0;
// Button L&R interrupt service routine
void buttonL_isr();
void buttonR_isr();

int main() {

   lcd.init();

   // Button L&R is connected between the pin and 3.3 V, we therefore need to turn on the internal pull-down resister
    buttonL.mode(PullDown);
    buttonR.mode(PullDown);
    // It will return 0 by default and a 1 when pressed i.e. cause a rising edge
    buttonL.rise(&buttonL_isr);
    buttonR.rise(&buttonR_isr);
    
    int p=20, q=3;      //位置
    int a=2, b=1;       //步长
    
   while(1) {
       
       lcd.clear();
       lcd.printString("Hello!",p,q);
       
       if (g_buttonL_flag) {
            p=p-a;
            q=q-b;
            g_buttonL_flag = 0;  // if it has, clear the flag
        }
        
        if (g_buttonR_flag) {
            p=p+a;
            q=q+b;
            g_buttonR_flag = 0;  // if it has, clear the flag
        }
        
       lcd.refresh();         
       wait(0.1);
   }
   
}

void buttonL_isr()
{
    g_buttonL_flag = 1;   // set flag in ISR
}

void buttonR_isr()
{
    g_buttonR_flag = 1;   // set flag in ISR
}