leetcode-0145 二叉树的后序遍历

https://leetcode.cn/problems/binary-tree-postorder-traversal/

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Deque<TreeNode> stack = new LinkedList<>();
        TreeNode prevAccess = null;
        while( root != null || !stack.isEmpty()){
            while( root != null){
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();  
            if(root.right == null || root.right == prevAccess){
                res.add(root.val);
                prevAccess = root;
                root = null;
            } else{
                stack.push(root);
                root = root.right;
            }        
        }
        return res;        
    }    
    // public List<Integer> postorderTraversal(TreeNode root) {
    //     List<Integer> res = new ArrayList<Integer>();
    //     accessTree(root, res);
    //     return res;
    // }
    public void accessTree(TreeNode root, List<Integer> res){
        if(root == null){
            return;
        }
        
        accessTree(root.left, res);        
        accessTree(root.right, res);
        res.add(root.val);
    }    
}