## Java 136 bytes##


**Without using String (136 bytes)**

    public static void main(String[] args) {
        for(int i = 0 ,k=1; i>-1; i+=k){
            for(int c= 65, d = 1; c>64;c+=d){
                d = d>-1 & c < 65+i?1:-1;
                System.out.print((char)c+(c+d<65?"\n":""));
            }
            k = k>-1 & i < 25?1:-1;
        }
    }
  
**Codegolfed**

    for(int i=0,k=1;i>-1;i+=k){for(int c=65,d=1;c>64;c+=d){d=d>-1&c<65+i?1:-1;System.out.print((char)c+(c+d<65?"\n":""));}k=k>-1&i<25?1:-1;}

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**With String(173 bytes)**

    String a="abcdefghijklmnopqrstuvwxyz";
    for(int i = 1 ,k=1; i>0; i+=k==1?1:-1){
        System.out.println(a.substring(0,i)+new StringBuilder(a).reverse().substring(27-i));
        k = k>-1 & i < 26?1:-1;
    }

**Codegolfed**

    String a="abcdefghijklmnopqrstuvwxyz";for(int i=1,k=1;i>0;k=k>-1&i<26?1:-1,System.out.println(a.substring(0,i)+new StringBuilder(a).reverse().substring(27-i)),i+=k==1?1:-1);


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**Thanks to [manatwork](https://codegolf.stackexchange.com/users/4198/manatwork)**