二叉树的遍历

节点结构：

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

一、先序遍历

public class PreOrder {
	ArrayList<TreeNode> list = new ArrayList<>();
	public void preOrder(TreeNode root) {
		if(root == null)
			return;
		list.add(root);
		preOrder(root.left);//左边
		preOrder(root.right);//右边
	}
}

二、中序遍历

public class InOrder {
	ArrayList<TreeNode> list = new ArrayList<>();
	public void inOrder(TreeNode root) {
		if(root == null)
			return;
		inOrder(root.left);//左边
		list.add(root);
		inOrder(root.right);//右边
	}
}

三、后序遍历

public class PostOrder {
	ArrayList<TreeNode> list = new ArrayList<>();
	public void postOrder(TreeNode root) {
		if(root == null)
			return;
		postOrder(root.left);//左边
		postOrder(root.right);//右边
		list.add(root);
	}
}

四、层序遍历

public class CengOrder {
	ArrayList<TreeNode> list = new ArrayList<>();
	LinkedList<TreeNode> queue = new LinkedList<>();//队列
	public void cengOrder(TreeNode root) {
		if(root != null) {
			queue.add(root);
			int num = 1;
			while(num != 0) {
				int temp = 0;
				for(int i = 0; i< num;i++) {
					TreeNode node = queue.poll();
					if(node.left != null) {
						queue.add(node.left);
						temp++;
					}
					if(node.right != null) {
						queue.add(node.right);
						temp++;
					}
					list.add(node);
				}
				num = temp;	
			}
		}
	}
}