145. Binary Tree Postorder Traversal

Given a binary tree, return the postorder traversal of its nodes' values.

 

Solution 1:

class Solution {

    public List<Integer> postorderTraversal(TreeNode root) {

    　　List<Integer> list = new ArrayList<>();

    　　if (root == null)

　　　　　return list;

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

    　　stack.push(root);

    　　while(!stack.empty()){

        　　root = stack.pop();

        　　list.add(0, root.val);

        　　if(root.left != null) 

            　　stack.push(root.left);

        　　if(root.right != null) 

            　　stack.push(root.right);

    　　}

    　　return list;

  }

}

 

Solution 2:

class Solution {

    List<Integer> result = new ArrayList<>();

    public List<Integer> postorderTraversal(TreeNode root) {     

        helper(root);                    

        return result;

    }

    

    private void helper(TreeNode root) {

         if (root == null)

            return;

         if (root.left != null)

             helper(root.left);

         if (root.right != null)

             helper(root.right);

         result.add(root.val);       

    }

}