How to reverse the order of elements in media queries?

Question

I have two divs, left and right, but when screen is less than for example 500px then left div become bottom div and right div becomes top div, div first in DOM should displays as second and second div as first.

I use display: flex, then divs are reversed but displays inline and ignore 100% width. What is wrong?

HTML

<div class='nav'>
    <div class='container'>
        <div class='left_bottom'>
            LEFT/BOTTOM
        </div>
        <div class='right_top'>
            RIGHT/TOP
        </div>
    </div>
</div>

CSS

.nav{
    background-color: red;
}
.container{
    width: 100%;
}
.left_bottom{
    padding: 15px 0 15px 0;
    background-color: green;
    float: left;
    width: 33.33%;
    text-align: center;
}
.right_top{
    padding: 15px 0 15px 0;
    background-color: blue;
    float: right;
    width: 66.66%;
    text-align: center;
}
@media (max-width: 500px){
    .left_bottom{
        float: left;
        width: 100%;
    }
    .right_top{
        float: left;
        width: 100%;
    }
    .container{
        display: flex;
        flex-direction: row-reverse;
    } 
}

JS FIDDLE

Answer 1

I suggest to use flexbox for all the layout (including desktop and mobile).

For desktop:

.container{
    display: flex;
}

For mobile (option 1):

@media (max-width: 500px){
    .container{
        flex-direction: column; /*display as column*/
    } 
    .left_bottom{
        order: 2; /*reorder to bottom*/
        width: 100%; /*make full width*/
    }
    .right_top{
        order: 1; /*reorder to top*/
        width: 100%; /*make full width*/
    }
}

jsFiddle

For mobile (option 2):

@media (max-width: 500px){
    .container{
        flex-direction: column-reverse; /*column & reverse*/
    } 
    .left_bottom, .right_top{
        width: 100%; /*make full width*/
    }
}

jsFiddle

Answer 2

Try this, it should work the way you wanted. let me know any issues.

.nav{
    background-color: red;
}
.container{
    width: 100%;
}
.left_bottom{
    padding: 15px 0 15px 0;
    background-color: green;
    float: left;
    width: 33.33%;
    text-align: center;
}
.right_top{
    padding: 15px 0 15px 0;
    background-color: blue;
    float: right;
    width: 66.66%;
    text-align: center;
}
@media (max-width: 500px)
    {
        .left_bottom{
            float: left;
            width: 100%;
        }
        .right_top{
            float: left;
            width: 100%;
        }
        
    }

    <div class='nav'>
        <div class='container'>
          <div class='right_top'>
                RIGHT/TOP
            </div>  
          <div class='left_bottom'>
                LEFT/BOTTOM
            </div>
            
        </div>
    </div>