import java.util.ArrayList;
import java.util.List;
import java.util.Stack;
public class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        if(root == null)
            return res;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode pre = null;
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode curr = stack.peek();
            if((curr.left == null && curr.right == null) ||
                    (pre != null && (pre == curr.left || pre == curr.right))){
                res.add(curr.val);
                pre = curr;
                stack.pop();
            }else{
                if(curr.right != null) stack.push(curr.right);
                if(curr.left != null) stack.push(curr.left);
            }
        }
        return res;
    }
}

后序遍历，左右根，左右节点都为空，添加根元素，当前节点不完空，不是左右节点，添加元素