var a = document.getElementsByTagName('div');
var b = false;
setInterval(function() {
for (var i = 0; i < 6; i++)
a[i].style.display = ((i & 1) ^ b) ? 'block' : 'none';
b = !b;

}, 1000);

j++; for (var i = 0; i < 6; i++) o[i].style.display = [['block', 'none'], ['none', 'block']][j & 1][i & 1];