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 MAX_LEN 100

void get_substring(char *source, int start, int end, char *dest) {
    int len = strlen(source);
    if (start < 0 || start >= len || end < 0 || end >= len || start > end) {
        return;
    }
    int i, idx = 0;
    for (i = start; i <= end; i++) {
        dest[idx++] = source[i];
    }
    dest[idx] = '\0';
}

int main() {
    char s1[MAX_LEN], s2[MAX_LEN], s3[MAX_LEN];
    char sub1[MAX_LEN], sub2[MAX_LEN], sub3[MAX_LEN];
    char new_str[3 * MAX_LEN];
    printf("Enter three strings:\n");
    scanf("%s %s %s", s1, s2, s3);
    get_substring(s1, 2, 4, sub1);
    get_substring(s2, 1, 3, sub2);
    get_substring(s3, 3, 5, sub3);
    strcpy(new_str, sub1);
    strcat(new_str, sub2);
    strcat(new_str, sub3);
    printf("New string: %s\n", new_str);
    return 0;
}