用C语言

好的，下面是同样实现的 C 语言版本：

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

char *process_string(char *s) {
    char *res = NULL;
    char *sub = NULL;
    int i = 0;
    while (s[i]) {
        char c = s[i];
        if (isalnum(c)) {
            if (!sub) {
                sub = (char *)malloc(sizeof(char) * 2);
                sub[0] = c;
                sub[1] = '\0';
            }
            else {
                int len = strlen(sub);
                sub = (char *)realloc(sub, sizeof(char) * (len + 2));
                sub[len] = c;
                sub[len+1] = '\0';
            }
        }
        else {
            if (sub) {
                free(sub);
            }
            return "<invalid input string>";
        }
        if (!s[i+1] || c != s[i+1]) {
            int len = strlen(sub);
            int j;
            for (j = 0; j < len; j++) {
                if (j == 0 || sub[j] > sub[j-1]) {
                    continue;
                }
                else {
                    free(sub);
                    sub = NULL;
                    break;
                }
            }
            if (sub) {
                if (!res || strncmp(sub, res, strlen(sub)) == 0) {
                    if (!res) {
                        res = (char *)malloc(sizeof(char) * (len + 1));
                        strcpy(res, sub);
                    }
                    else if (strlen(sub) > strlen(res)) {
                        res = (char *)realloc(res, sizeof(char) * (len + 1));
                        strcpy(res, sub);
                    }
                }
                else {
                    res = (char *)realloc(res, sizeof(char) * (strlen(sub) + 1));
                    strncpy(res, sub, strlen(sub));
                    res[strlen(sub)] = '\0';
                }
            }
            else {
                free(sub);
                return "<invalid input string>";
            }
            free(sub);
            sub = NULL;
        }
        i++;
    }
    return res;
}

int main() {
    char s1[] = "aabbccdd";
    char s2[] = "007799aabbccddeeff113355zz";
    char s3[] = "1234.89898";
    char s4[] = "abcdefabcdefabcdefaaaaaaaaaaaaaabbbbbbbddddddee";
    printf("%s\n", process_string(s1)); // abcdabcd
    printf("%s\n", process_string(s2)); // 013579abcdefz013579abcdefz
    printf("%s\n", process_string(s3)); // <invalid input string>
    printf("%s\n", process_string(s4)); // abcdefabcdefabcdefabdeabdeabdabdabdabdabaaaaaaa
    return 0;
}

同样，您可以把这段代码复制到您的编程环境中，然后编译运行，即可得到处理后的字符串。