LeetCode 590. N叉樹的後序遍歷 (遞歸、迭代實現)

N叉樹的後序遍歷
遞歸：

/*
// Definition for a Node.
class Node {
public:
    int val;
    vector<Node*> children;

    Node() {}

    Node(int _val) {
        val = _val;
    }

    Node(int _val, vector<Node*> _children) {
        val = _val;
        children = _children;
    }
};
*/

class Solution {
public:
    vector<int> ans;
    vector<int> postorder(Node* root) {
        dfs(root);
        return ans;
    }
    void dfs(Node* root){
        if(!root){
            return ;
        }
        for(Node* child:root->children){
            dfs(child);
        }
        ans.push_back(root->val);
    }
};

迭代：

/*
// Definition for a Node.
class Node {
public:
    int val;
    vector<Node*> children;

    Node() {}

    Node(int _val) {
        val = _val;
    }

    Node(int _val, vector<Node*> _children) {
        val = _val;
        children = _children;
    }
};
*/

class Solution {
public:
    vector<int> postorder(Node* root) {
        stack<Node*> call;
        vector<int> ans;
        if(root){
            call.push(root);
        }
        while(!call.empty()){
            Node* par = call.top();
            call.pop();
            if(par){
                call.push(par);
                call.push(nullptr);
                for(int i=par->children.size()-1;i>=0;i--){
                    call.push(par->children[i]);
                }
            }else{
                ans.push_back(call.top()->val);
                call.pop();
            }
        }
        return ans;
    }
};