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) {
        // write code here
        // 解题思路：后序遍历是左、根、右
        List<Integer> list = new ArrayList<>();
        back(root, list);

        int[] a = new int[list.size()];

        for (int i = 0; i < list.size(); i++) {
            a[i] = list.get(i);
        }

        return a;
    }

    private void back(TreeNode root, List<Integer> list) {
        if (root == null) {
            return;
        }

        back( root.left, list) ;
        back( root.right, list) ;
        list.add(root.val);
    }
}