LeetCode126|二叉树的后续遍历

输入: [1,null,2,3]  
   1
    \
     2
    /
   3 
输出: [3,2,1]

import java.util.ArrayList;
import java.util.List;

public class PostorderTraversalTest2 {
    public static void main(String[] args) {
        TreeNode t1 = new TreeNode(1);
        TreeNode t2 = new TreeNode(2);
        TreeNode t3 = new TreeNode(3);
        t1.right = t2;
        t2.left = t3;
        List<Integer> list = postorderTraversal(t1);
        System.out.println("list = " + list);

    }

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

    public static List<Integer> postorderTraversal(TreeNode root) {
        if (root == null) {
            return list;
        }
        dfs(root);
        return list;
    }

    private static void dfs(TreeNode root) {
        if (root.left != null) {
            dfs(root.left);
        }
        if (root.right != null) {
            dfs(root.right);
        }
        list.add(root.val);
    }
}