can cops come into your house without a warrant?

[Alex S]

umm i here people say all the time that cops can come into your house with probably cause right. but all they have to do is walk up to the door and say they have cause to search or come in. i think cops use there rights a little to much i see cops all the time whip u-turns in intersections where theres no u-turns and they dont use there blinker all the time i just want to protect myself when this case happens