题解 | #二叉树的后序遍历#

import java.util.*;

public class Solution {
    public int[] postorderTraversal (TreeNode root) {
        if (root == null){
            return new int[0];
        }
        List<Integer> ll = new ArrayList<>();
        TreeNode tn = root;
        if (root.left != null) {
            tn = root.left;
            int[] left = postorderTraversal(tn);
            for (int j = 0; j < left.length; j++){
                ll.add(left[j]);
            }
        }
        if (root.right != null) {
            tn = root.right;
            int[] right = postorderTraversal(tn);
            for (int j = 0; j < right.length; j++)
                ll.add(right[j]);
        }
        ll.add(root.val);
        int[] result = new int[ll.size()];
        for (int i = 0; i < ll.size(); i++) {
            result[i] = ll.get(i);
        }
        return result;
    }
}