LeetCode_589_N叉树的前序遍历

题目描述：

递归法：跟二叉树的前序遍历类似。

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

    Node() {}

    Node(int _val, vector<Node*> _children) {
        val = _val;
        children = _children;
    }
};
*/
class Solution {
public:
    vector<int> res;
    vector<int> preorder(Node* root) {
        if(root==NULL)
            return res;
        res.push_back(root->val);
        for(Node* child:root->children){
            preorder(child);
        }
        return res;
    }
};

递推算法：采用stack来完成

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

    Node() {}

    Node(int _val, vector<Node*> _children) {
        val = _val;
        children = _children;
    }
};
*/
class Solution {
public:
    vector<int> preorder(Node* root) {
        if(root==NULL) return {};
        stack<Node*> stk;
        vector<int> res;
        stk.push(root);
        while(!stk.empty()) {
            Node* temp=stk.top();
            stk.pop();
            for(int i=temp->children.size()-1;i>=0;i--) stk.push(temp->children[i]);//倒放子节点
            res.push_back(temp->val);
        }
        return res;
    }
};

还有一种递推形式的先序遍历，未完待续…