while(x>=0&&y>=0){
        rev1(x,y);if(x<0||y<0)break;ans=(ans-C(x+y,y)%mod+mod)%mod;
        rev2(x,y);if(x<0||y<0)break;ans=(ans+C(x+y,y)%mod)%mod;
    }
    x=n+m+1,y=n;
    while(x>=0&&y>=0){
        rev2(x,y);if(x<0||y<0)break;ans=(ans-C(x+y,y)%mod+mod)%mod;
        rev1(x,y);if(x<0||y<0)break;ans=(ans+C(x+y,y)%mod)%mod;
    }

记得转换的时候及时break。 `