Likou - preorder traversal, inorder traversal, postorder traversal of binary tree

一、题目描述：给你二叉树的根节点 root ,返回它节点值的 前序 遍历.

1.递归：

class Solution {
    
    List<Integer> list = new ArrayList<>();
    public List<Integer> preorderTraversal(TreeNode root) {
    
        if(root == null) return list;
        list.add(root.val);
        preorderTraversal(root.left);
        preorderTraversal(root.right);
        return list;
    }
}

2.非递归

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

二、中序遍历
递归：

class Solution {
    
    List<Integer> list = new ArrayList<>();
    public List<Integer> inorderTraversal(TreeNode root) {
    
        if(root == null) return list;
        inorderTraversal(root.left);
        list.add(root.val);
        inorderTraversal(root.right);
        return list;
    }
}

非递归：

class Solution {
    
    public List<Integer> inorderTraversal(TreeNode root) {
    
        List<Integer> list = new ArrayList<>();
        if(root == null) return list;
        Stack<TreeNode> stack = new Stack<>();
        while(root != null || !stack.empty()) {
    
            while(root!= null) {
    
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            list.add(root.val);
            root = root.right;
        }
        return list;
    }
}

三、后序遍历

class Solution {
    
    List<Integer> list = new ArrayList<>();
    public List<Integer> postorderTraversal(TreeNode root) {
    
        if(root == null) return list;
        postorderTraversal(root.left);
        postorderTraversal(root.right);
        list.add(root.val);
        return list;
    }
}