lintcode：Binary Tree Postorder Traversal 二叉树的后序遍历

题目：

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]

挑战

你能使用非递归实现么？

解题：

递归程序好简单

Java程序：

/**
 * 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 Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Postorder in ArrayList which contains node values.
     */
    public ArrayList<Integer> postorderTraversal(TreeNode root) {
        // write your code here
        ArrayList<Integer> res = new ArrayList<Integer>();
        res = postorder(res,root);
        return res;
    }
    public ArrayList<Integer> postorder(ArrayList<Integer> res,TreeNode root){
        if(root==null)
            return res;
        if(root.left!=null)
            res = postorder(res,root.left);
        if(root.right!=null)
            res = postorder(res,root.right);
        res.add(root.val);
        return res;
    }
}

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