http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=0570 この問題に出てくるような、ジグザグ数を探すというのをC言語でやってみた感じのやつ。

zigzag.c

#include <stdio.h>
#include <stdbool.h>
#define ZIG 1
#define NONE 0
#define ZAG -1

bool is_zigzag(char* s) {
        int mode = NONE;
        char prev = '\0';
        for(int i = 0; s[i] != '\0'; i++) {
                if(prev != '\0') {
                        if(prev < s[i] && mode != ZIG) {
                                mode = ZIG;
                        } else if(s[i] < prev && mode != ZAG) {
                                mode = ZAG;
                        } else {
                                return false;
                        }
                }
                prev = s[i];
        }
        return true; // this number is zig-zag
}

int main(void) {
        char num[80];
        printf("input number: ");
        scanf("%s", num);
        if(is_zigzag(num)) {
                printf("this number is zig-zag number.\n");
        } else {
                printf("this number is not zig-zag number.\n");
        }
        return 0;
}