递归

/**
 * 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<Integer>();
        accessTree(root,res);
        return res;
    }

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

栈（复习）

/**
 * 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<Integer>();
        Deque<TreeNode> stack = new LinkedList<TreeNode>();
        // 新增pre记录
        TreeNode preAccess = null;
        while(root!=null||!stack.isEmpty()){
            while(root!=null){
                stack.push(root);
                root=root.left;
            }
            root=stack.pop();
            if(root.right==null||root.right==preAccess){
                res.add(root.val);
                preAccess=root;
                root=null;
            }else{
                stack.push(root);
                root=root.right;
            }
        }
        return res;
    }
}