DFs:

public static void top(Node root,Map m,int w,int h){

    if(root == null) return;

    if(!m.containsKey(w))  
        m.put(w, new int[]{h,root.data});
    else if(m.get(w)[0] > h)
        m.put(w, new int[]{h,root.data});

    top(root.left, m,w-1,h+1);
    top(root.right, m, w+1,h+1);

}

public static void topView(Node root) {

    Map<Integer,int[]> m = new TreeMap<Integer,int[]>();
    top(root,m,0,0);
    for(int k : m.keySet()){
        System.out.print(m.get(k)[1] + " ");
    }
}