BM25 题解 | #二叉树的后序遍历#

解题思路：

递归，后序遍历，就是vist(T) 放在最后

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 {
    /**
     * 
     * @param root TreeNode类 
     * @return int整型一维数组
     */
    public int[] postorderTraversal (TreeNode root) {
        List<Integer> res = new ArrayList<>();
        postOrderNode(root, res);
        int[] arr =new int[res.size()];
        for(int i=0; i<res.size(); i++) {
            arr[i] = res.get(i);
        }
        return arr;
    }

    private void postOrderNode(TreeNode root, List<Integer> res) {
        if(root == null) return ;
        postOrderNode(root.left, res);
        postOrderNode(root.right, res);
        res.add(root.val);
    }
}