题解 | #二叉树的后序遍历#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param root TreeNode类
     * @return int整型一维数组
     */
    List<Integer> list = new ArrayList<>();
    public int[] postorderTraversal (TreeNode root) {
        // write code here
        postorder(root);
        int[] ans = new int[list.size()];
        int k = 0;
        for(int num : list){
            ans[k++] = num;
        }
        return ans;
    public void postorder(TreeNode root){
        if(root == null){
            return ;
        }
        postorder(root.left);
        postorder(root.right);
        list.add(root.val);
    }
}