用c++输入三个字符串，通过指针读取各个字符串的子串（子串是指字符串中连续的一小部分），把它们合并成一个新字符串

#include <stdio.h>
#include <string.h>

#define MAXSIZE 50

void getSubstr(char *str, int start, int end, char *substr);
//获取字符串str中 [start, end) 区间内的子串

int main(void)
{
    char str1[MAXSIZE], str2[MAXSIZE], str3[MAXSIZE];
    char *p1, *p2, *p3;
    char substr1[MAXSIZE], substr2[MAXSIZE], substr3[MAXSIZE];
    char newstr[MAXSIZE*3+1];
    
    printf("Enter string 1: ");
    fgets(str1, MAXSIZE, stdin);
    printf("Enter string 2: ");
    fgets(str2, MAXSIZE, stdin);
    printf("Enter string 3: ");
    fgets(str3, MAXSIZE, stdin);
    
    //获取各个字符串的子串
    getSubstr(str1, 0, strlen(str1)-1, substr1); //整个字符串作为子串
    getSubstr(str2, 1, strlen(str2)-2, substr2); //去掉末尾的换行符
    getSubstr(str3, 3, 6, substr3);
    
    //合并成新字符串
    p1 = substr1;
    p2 = substr2;
    p3 = substr3;
    while(*p1 != '\0')
    {
        *newstr++ = *p1++;
    }
    while(*p2 != '\0')
    {
        *newstr++ = *p2++;
    }
    while(*p3 != '\0')
    {
        *newstr++ = *p3++;
    }
    *newstr = '\0'; //注意加上字符串结束符
    
    printf("New string: %s", newstr-MAXSIZE*3); //输出时回到newstr的起始位置
    
    return 0;
}

void getSubstr(char *str, int start, int end, char *substr)
{
    int i;
    for(i=start; i<end; i++)
    {
        substr[i-start] = str[i];
    }
    substr[i-start] = '\0';
}