LeetCode 144 Binary Tree Preorder Traversal（递归+非递归）

题目链接：点击这里

递归：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void preorder(TreeNode* r, vector<int> &v)
    {
        if(r!=NULL)
        {
            v.push_back(r->val);
            preorder(r->left, v);
            preorder(r->right, v);
        }
    }
    
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> pre;
        preorder(root, pre);
        return pre;
    }
};

非递归：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        stack<TreeNode*> st;    //使用栈来辅助
        vector<int> pre;        //存储先序序列
        st.push(root);          //根节点首先进栈
        while(!st.empty())
        {
            TreeNode* p = st.top();
            st.pop();
            if(p!=NULL)
            {
                pre.push_back(p->val);
                st.push(p->right);    //右儿子先进入
                st.push(p->left);     //左儿子后进入
            }
        }
        return pre;
    }
};