Leetcode(145) Binary Tree Postorder Traversal

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Description:

Given a binary tree, return the postorder traversal of its nodes’ values.

解法：

递归解题，具体代码如下：

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class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> output = new ArrayList();
        helper(root,output);
        return output;
    }
    void helper(TreeNode root,List<Integer> output){
        if(root == null){
            return;
        }
        helper(root.left,output);
        helper(root.right,output);
        output.add(root.val);
    }
}