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Java 157 bytes (without using String)

Thanks to manatwork

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