Binary Tree Postorder Traversal

http://bookshadow.com/weblog/2015/01/19/leetcode-binary-tree-postorder-traversal/

http://bookshadow.com/weblog/2015/01/19/binary-tree-post-order-traversal/

记不住啊总，不如试试双栈法，借用如上链接提示，反向preorder

 

要注意压入栈的时候子节点左右顺序

public class Solution {

    public ArrayList<Integer> postorderTraversal(TreeNode root) {

        ArrayList<Integer> res = new ArrayList<Integer>();

        if(root==null) return res;

        Stack<TreeNode> st = new Stack<TreeNode>();

        Stack<TreeNode> out= new Stack<TreeNode>();

        st.push(root);

        while(!st.isEmpty()){

            TreeNode n= st.pop();

            out.push(n);

            if(n.left!=null){

                st.push(n.left);

            }

            if(n.right!=null){

                st.push(n.right);

            }

        }

        while(!out.isEmpty()){

            TreeNode o = out.pop();

            res.add(o.val);

        }

        return res;

    }

}