Forward button - back on a slide

Question

Well, I slided it and it’s working. But it advances automatically, so I want to know how I can add a button to pass and to return to the previous slide.

function simpleSlider(){
	var sliderActive = $("#slider .sliderActive");
	var sliderNext   = sliderActive.next().length ? sliderActive.next() : $("#slider li:first");
	sliderNext.addClass('sliderActive').fadeIn();
	sliderActive.removeClass('sliderActive').fadeOut();
}
$(function(){
	$("#slider li:first").fadeIn();
	setInterval("simpleSlider()", 3000);
});

#slider {
    list-style:none;
    width:800px;
    height:700px;
    margin:0 auto;
    padding:0;
    overflow: hidden;
    position: relative;
}
#slider li {
    position: absolute;
    z-index: 0;
    display:none;
}
#slider li.sliderActive {
    z-index: 1;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
<ul id="slider">
    <li><div class="box_inside box_crm">
        <span class="icon_serv_crm"></span>
        <h3>Lorem ipsum dolor sit amet</h3>
        <p class="principal"><strong>Lorem ipsum dolor sit amet, consectetur adipiscing elit.</strong></p>
        <p>Maecenas sagittis, lorem non imperdiet faucibus, neque turpis porta velit, ultrices ullamcorper elit tellus in nisl. Maecenas enim felis, sollicitudin convallis tristique at, ultrices quis mauris. Pellentesque et fringilla nunc. Phasellus magna metus, placerat eget tincidunt non, dictum non nisi. </p>    
        <div class="content_btn center" style="width:100%">
            <a href="#" class="btn">Saiba mais</a>        
        </div>
    </div></li>
    <li><div class="box_inside box_landing">
        <span class="icon_serv_land"></span>
        <h3>Lorem ipsum dolor sit amet</h3>
        <p class="principal"><strong>t facilisis et sapien a auctor. Cras in iaculis eros, ac tincidunt mi. Aenean auctor ultricies dolor, sed dictum lectus iaculis sed. Nullam lobortis</strong></p>
        <p>t facilisis et sapien a auctor. Cras in iaculis eros, ac tincidunt mi. Aenean auctor ultricies dolor, sed dictum lectus iaculis sed. Nullam lobortis</p>    
        <div class="content_btn center" style="width:100%">
            <a href="http://localhost/" class="btn">Saiba mais</a>        
        </div>
    </div></li>
</ul>

Answer 1

I decided as follows:

function simpleSlider(type){
        	var sliderActive = $("#slider .sliderActive");
        		if(type == 'prev') {
        			var sliderPrev   = sliderActive.prev().length ? sliderActive.prev() : $("#slider li:last");
	                    sliderPrev.addClass('sliderActive').fadeIn();
        		} else {
	                    var sliderNext   = sliderActive.next().length ? sliderActive.next() : $("#slider li:first");
	                    sliderNext.addClass('sliderActive').fadeIn();
	                }
	                sliderActive.removeClass('sliderActive').fadeOut();
            }
        $(function(){
            $("#slider li:first").fadeIn();
            
        });

#slider {
    list-style:none;
    width:800px;
    height:700px;
    margin:0 auto;
    padding:0;
    overflow: hidden;
    position: relative;
}
#slider li {
    position: absolute;
    z-index: 0;
    display:none;
}
#slider li.sliderActive {
    z-index: 1;
}

<div class="buttons">
 <a href="javascript:simpleSlider('prev')"><span>prev</span></a>
 <a href="javascript:simpleSlider('next')"><span>next</span></a>
</div>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
<ul id="slider">
    <li><div class="box_inside box_crm">
        <span class="icon_serv_crm"></span>
        <h3>Lorem ipsum dolor sit amet</h3>
        <p class="principal"><strong>Lorem ipsum dolor sit amet, consectetur adipiscing elit.</strong></p>
        <p>Maecenas sagittis, lorem non imperdiet faucibus, neque turpis porta velit, ultrices ullamcorper elit tellus in nisl. Maecenas enim felis, sollicitudin convallis tristique at, ultrices quis mauris. Pellentesque et fringilla nunc. Phasellus magna metus, placerat eget tincidunt non, dictum non nisi. </p>    
        <div class="content_btn center" style="width:100%">
            <a href="#" class="btn">Saiba mais</a>        
        </div>
    </div></li>
    <li><div class="box_inside box_landing">
        <span class="icon_serv_land"></span>
        <h3>Lorem ipsum dolor sit amet</h3>
        <p class="principal"><strong>t facilisis et sapien a auctor. Cras in iaculis eros, ac tincidunt mi. Aenean auctor ultricies dolor, sed dictum lectus iaculis sed. Nullam lobortis</strong></p>
        <p>t facilisis et sapien a auctor. Cras in iaculis eros, ac tincidunt mi. Aenean auctor ultricies dolor, sed dictum lectus iaculis sed. Nullam lobortis</p>    
        <div class="content_btn center" style="width:100%">
            <a href="http://localhost/" class="btn">Saiba mais</a>        
        </div>
    </div></li>
</ul>