68. 二叉树的后序遍历

给出一棵二叉树，返回其节点值的后序遍历。

样例
给出一棵二叉树 {1,#,2,3},
   1
    \
     2
    /
   3

返回 [3,2,1]

代码

import java.util.ArrayList;
import java.util.List;

import LintClass.TreeNode;

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */


public class PostorderTraversal_68 {
    /*
     * @param root: A Tree
     * @return: Postorder in ArrayList which contains node values.
     */
    List<Integer> output = new ArrayList<Integer>();
    public List<Integer> postorderTraversal(TreeNode root) {
        // write your code here
        TreeNode cur = root;
        
        if(cur == null)
            return output;
        
        if(cur.left != null) {
            postorderTraversal(cur.left);
            //output.add(cur.left.val);
        }
        
        if(cur.right != null) {
            postorderTraversal(cur.right);
            //output.add(cur.right.val);
        }
        
        output.add(cur.val);
        
        return output;
    }
    
    public static void main(String[] args) {
        TreeNode root = new TreeNode(1);
        TreeNode left_1 = new TreeNode(4);
        TreeNode left_2_right = new TreeNode(5);
        TreeNode right_1 = new TreeNode(2);
        TreeNode right_2_left = new TreeNode(3);
        root.left = left_1;
        root.left.right = left_2_right;
        root.right = right_1;
        root.right.left = right_2_left;
        
        // left, right, root
        PostorderTraversal_68 obj = new PostorderTraversal_68();
        System.out.print(obj.postorderTraversal(root));;
    }
}