LeetCode 145. 二叉树的后序遍历

class Solution {
public:
    
    vector postorderTraversal(TreeNode* root) {
        vector ans;
        stack stk;
        while(root||stk.size()){
            while(root){
                ans.push_back(root->val);
                stk.push(root);
                root=root->right;
            }
           
            root=stk.top()->left;
            stk.pop();
        }
        reverse(ans.begin(),ans.end());
        return ans;
    }
   
};

class Solution {
public:
    vector ans;
    vector postorderTraversal(TreeNode* root) {
        dfs(root);
        return ans;
    }
    void dfs(TreeNode* root){
        if(!root) return ;
        dfs(root->left);
        dfs(root->right);
        ans.push_back(root->val);
    }
};