Leetcode144/94/145-经典二叉树前序中序后序遍历

Leetcode144-前序遍历

• 递归版

  List<Integer> res = new ArrayList<>();

    public List<Integer> preorderTraversal(TreeNode root) {
        preorder(root);
        return res;
    }

    public void preorder(TreeNode root) {
        if (root == null) {
            return;
        }
        res.add(root.val);
        preorder(root.left);
        preorder(root.right);
    }

• 迭代版

   public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null){
            return result;
        }
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()){
            TreeNode node = stack.pop();
            result.add(node.val);
            if (node.right != null){
                stack.push(node.right);
            }
            if (node.left != null){
                stack.push(node.left);
            }
        }
        return result;
    }

Leetcode94-中序遍历

• 递归版

    List<Integer> res=new ArrayList<>();

    public List<Integer> inorderTraversal(TreeNode root) {
        inorder(root);
        return res;
    }

    public void inorder(TreeNode root){
        if(root==null){
            return;
        }

        inorder(root.left);
        res.add(root.val);
        inorder(root.right);
    }

• 迭代版

 public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null){
            return result;
        }
        Stack<TreeNode> stack = new Stack<>();
        TreeNode cur = root;
        while (cur != null || !stack.isEmpty()){
           if (cur != null){
               stack.push(cur);
               cur = cur.left;
           }else{
               cur = stack.pop();
               result.add(cur.val);
               cur = cur.right;
           }
        }
        return result;
    }

Leetcode145-后序遍历

• 递归版

  List<Integer> res = new ArrayList<Integer>();
    public List<Integer> postorderTraversal(TreeNode root) {
        postorder(root);
        return res;
    }


    public void postorder(TreeNode root){
        if(root==null){
            return;
        }
        postorder(root.left);
        postorder(root.right);
        res.add(root.val);
    }

• 迭代版

    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null){
            return result;
        }
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()){
            TreeNode node = stack.pop();
            result.add(node.val);
            if (node.left != null){
                stack.push(node.left);
            }
            if (node.right != null){
                stack.push(node.right);
            }
        }
        Collections.reverse(result);
        return result;
    }