树的定义

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

前序遍历

public void preorderStack(TreeNode root) {
		Stack<TreeNode> stack=new Stack<>();
		while(root!=null || !stack.isEmpty()){
			while(root!=null){
				stack.push(root);
				System.out.println(root.val);
				root=root.left;
			}
			root=stack.pop();
			root=root.right;
		}
	}
	public void preOrder(TreeNode root){
		if(root!=null){
			System.out.println(root.val);
			preOrder(root.left);
			preOrder(root.right);
		}
	}

中序遍历

	public void inorderStack(TreeNode root) {
		Stack<TreeNode> stack=new Stack<>();
		while(root!=null || !stack.isEmpty()){
			while(root!=null){
				stack.push(root);
				root=root.left;
			}
			root=stack.pop();
			System.out.println(root.val);
			root=root.right;
		}
	}
	public void inOrder(TreeNode root){
		if(root!=null){
		
			inOrder(root.left);
			System.out.println(root.val);
			inOrder(root.right);
		}
	}

后序遍历

public void  postorderStack(TreeNode root){
		Stack<TreeNode>stack=new Stack<>();
		Map<TreeNode, Boolean> map=new HashMap<>();
		stack.push(root);
		while(!stack.isEmpty()){
			TreeNode node=stack.peek();
			if(node.left!=null && !map.containsKey(node.left)){
				node=node.left;
				while(node!=null){
					if(map.containsKey(node))break;
					stack.push(node);
					node=node.left;
				}
				continue;
			}
			if(node.right!=null && !map.containsKey(node.right)){
				stack.push(node.right);
				continue;
			}
			TreeNode t=stack.pop();
			map.put(t, true);
			System.out.println(t.val);
		}
	}
	public void postOrder(TreeNode root){
		if(root!=null){
			postOrder(root.left);
			postOrder(root.right);
			System.out.println(root.val);
		}
	}