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
        Stack<TreeNode> stack = new Stack<>();
        ArrayList<Integer> list = new ArrayList<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            if(node != null)
            list.add(node.val);
            else continue;
            if (node.left != null)
                stack.push(node.left);
           if (node.right != null)
                stack.push(node.right);
            
        }
        Collections.reverse(list);
        int []arr = new int[list.size()];
        for (int i = 0 ; i < list.size() ; i++) {
            arr[i] = list.get(i);
        }
        return arr;
    }
}