前序遍历、中序遍历、后序遍历、层次遍历-蒲公英云

前序遍历、中序遍历、后序遍历、层次遍历

public class Tree { 
    public static void main(String[] args) { 
        TreeNode root = new TreeNode(1);
        root.left = new TreeNode(2);
        root.right = new TreeNode(3);
        root.left.left = new TreeNode(4);
        preOrder(root);
        System.out.println("");
        midOrder(root);
        System.out.println("");
        postOrder(root);
        System.out.println("");
        levOrder(root);
    }
    static void preOrder(TreeNode node) { 
        if (node == null) return;
        System.out.printf("%d ", node.val);
        preOrder(node.left);
        preOrder(node.right);
    }
    static void midOrder(TreeNode node) { 
        if (node == null) return;
        midOrder(node.left);
        System.out.printf("%d ", node.val);
        midOrder(node.right);
    }
    static void postOrder(TreeNode node) { 
        if (node == null) return;
        postOrder(node.left);
        postOrder(node.right);
        System.out.printf("%d ", node.val);
    }
    static void levOrder(TreeNode node) { 
        if (node == null) return;
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(node);
        while(!queue.isEmpty()) { 
            TreeNode tmp = queue.poll();
            System.out.printf("%d ", tmp.val);
            if (tmp.left != null) queue.offer(tmp.left);
            if (tmp.right != null) queue.offer(tmp.right);
        }
    }
}
class TreeNode { 
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int val) { 
        this.val = val;
    }
}