树的遍历（前序，中序，后续，栈与递归实现）

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);}}