LeetCode-145-Binary Tree Postorder Traversal

算法描述：

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

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [3,2,1]

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

解题思路：用栈模拟。

    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> results;
        if(root==nullptr) return results;
        stack<TreeNode*> stk;
        stk.push(root);
        while(!stk.empty()){
            TreeNode* temp = stk.top();
            stk.pop();
            results.insert(results.begin(),temp->val);
            if(temp->left) stk.push(temp->left);
            if(temp->right) stk.push(temp->right);
           
        }
        return results;
    }