算法通关模板-二叉树遍历(前序、中序、后序)递归、非递归

递归

中序遍历

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {

        List<Integer> list = new LinkedList<>();
        inorder(root,list);
        return list;
        
    }
    private void inorder(TreeNode root,List<Integer> list){
        if(root != null){
            inorder(root.left,list);
            list.add(root.val);
            inorder(root.right,list);
        }
    }
}

前序遍历

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new LinkedList<>();

        preorder(root,list);
        return list;
    }
    private void preorder(TreeNode root,List<Integer> list){
        if(root!=null){
            list.add(root.val);
            preorder(root.left,list);
            preorder(root.right,list);
        }
    }
}

后序遍历

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        
        List<Integer> list = new LinkedList<>();
        postorder(root,list);
        return list;
        
    }

    private void postorder(TreeNode root,List<Integer> list){
        if(root!=null){
            postorder(root.left,list);            
            postorder(root.right,list);
            list.add(root.val);
        }
    }
}

非递归

中序遍历

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        List<Integer> list = new LinkedList<>();

        while(root!=null || !stack.isEmpty()){
            if(root != null){
                stack.push(root);
                root = root.left;
            }else{
                root = stack.pop();
                list.add(root.val);
                root = root.right;
            }
        }

        return list;
    }
}

前序遍历

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        List<Integer> list = new LinkedList<>();

        while(root!=null || !stack.isEmpty()){
            if(root!=null){
                stack.push(root);
                list.add(root.val);
                root = root.left;
            }else{
                root = stack.pop();           
                root = root.right;
            }
        }
        return list;
    }
}

后序遍历

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        LinkedList<Integer> list = new LinkedList<>();

        while(root!=null || !stack.isEmpty()){
            if(root!=null){
                stack.push(root);
                list.addFirst(root.val);
                root = root.right;
                
            }else{
                root = stack.pop();
                root = root.left;
            }
        }
        return list;
    }
}