前序：根-左-右

中序：左-根-右

后序：左-右-根

实现：

class TreeNode1 {
    int val = 0;
    TreeNode1 left = null;
    TreeNode1 right = null;
    public TreeNode1(int val) {
        this.val = val;
    }
}

public class printTree {
	public static void main(String[] args) {
		TreeNode1 A=new TreeNode1(1);
		TreeNode1 B=new TreeNode1(2);
		TreeNode1 C=new TreeNode1(3);
		TreeNode1 D=new TreeNode1(4);
		TreeNode1 E=new TreeNode1(5);
		TreeNode1 F=new TreeNode1(6);
		A.left=B;A.right=C;
		B.left=D;B.right=E;
		C.left=null;C.right=F;
		D.left=null;D.right=null;
		E.left=null;E.right=null;
		F.left=null;F.right=null;
		System.out.print("前序遍历结果：");
		preTree(A);
		System.out.println();
		System.out.print("中序遍历结果：");
		inTree(A);
		System.out.println();
		System.out.print("后序遍历结果：");
		postTree(A);
	}
	static void preTree(TreeNode1 root){//前序遍历
		if(root==null){
			return;
		}
		System.out.print(root.val+" ");
		if(root.left!=null){
			preTree(root.left);
		}
		if(root.right!=null){
			preTree(root.right);
		}
	}
	static void inTree(TreeNode1 root){//中序遍历
		if(root.left!=null){
			inTree(root.left);
		}
		System.out.print(root.val+" ");
		if(root.right!=null){
			inTree(root.right);
		}
	}
	static void postTree(TreeNode1 root){//后序遍历
		if(root.left!=null){
			inTree(root.left);
		}
		if(root.right!=null){
			inTree(root.right);
		}
		System.out.print(root.val+" ");
	}
}