leetcode-145. 二叉树的后序遍历

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class Solution {
    List<Integer> result = new ArrayList<>();
    public List<Integer> postorderTraversal(TreeNode root) {
        dfs(root);
        return result;
    }

    public void dfs(TreeNode root){
        if(root == null){
            return;
        }

        if(root.left != null){
            dfs(root.left);
        }

        if(root.right != null){
            dfs(root.right);
        }

        if(root != null){
            result.add(root.val);
        }
    }
}