Binary Tree Postorder Traversal

Given binary tree {1,#,2,3},

1

2

/

3

return [3,2,1].

 

Note: Recursive solution is trivial, could you do it iteratively?

 

Solution: 1. Iterative way (stack). Time: O(n), Space: O(n).

2. Recursive solution. Time: O(n), Space: O(n).

3. Threaded tree (Morris). Time: O(n), Space: O(1).

You may refer to my blog for more detailed explanations:

http://www.cnblogs.com/AnnieKim/archive/2013/06/15/MorrisTraversal.html

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> postorderTraversal(TreeNode *root) {
        vector<int> res;
        postorderTraversalRe(root, res);
        return res;
    }
    
    void postorderTraversalRe(TreeNode* root, vector<int> &res)
    {
        if(!root) return;
        postorderTraversalRe(root->left, res);
        postorderTraversalRe(root->right, res);
        res.push_back(root->val);
    }
};