计算字符串中子串出现的次数。要求：用一个子函数subString()实现，参数为指向字符串和要查找的子串的指针，返回次数。

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

int subString(char *str, char *sub) {
    int count = 0;
    int len = strlen(sub);
    while ((str = strstr(str, sub)) != NULL) {
        count++;
        str += len;
    }
    return count;
}

int main() {
    char str[] = "hellohellohello";
    char sub[] = "ll";
    int count = subString(str, sub);
    printf("%d\n", count);  // 6
    return 0;
}