奇怪，感觉思路没错啊，时间复杂度也就 ${O(y-x)}$。

#include <stdio.h>
int main() {
    int x, y;
    scanf("%d%d", &x, &y);
    printf("%d\n", y);
    y -= x;
    while (x--)
        printf("1 ");
    for (int i = 2; i <= y; i += 2)
        printf("1 -1 ");
    return 0;
}