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
     */
    public int[] postorderTraversal (TreeNode root) {

        if (root == null) {
            return new int[0];
        }
        List<Integer> ans = new ArrayList<>();
        postOrder(root, ans);
        int[] res = new int[ans.size()];

        for (int i = 0; i < ans.size(); ++i) {
            res[i] = ans.get(i);
        }
        return res;
    }

    public void postOrder(TreeNode root, List<Integer> ans) {

        if (root == null) {
            return;
        }
        postOrder(root.left, ans);
        postOrder(root.right, ans);
        ans.add(root.val);
    }
}