题目链接:https://leetcode-cn.com/problems/binary-tree-preorder-traversal/
题目描述
给定一个二叉树,返回它的 前序 遍历。
示例:
输入: [1,null,2,3]
1
\
2
/
3
输出: [1,2,3]
进阶: 递归算法很简单,你可以通过迭代算法完成吗?
非递归
class Solution {
public:
vector<int> preorderTraversal(TreeNode* root) {
stack<TreeNode*> s;
vector<int> ret;
if(!root) return ret;
s.push(root);
while(!s.empty()){
root = s.top();
s.pop();
ret.push_back(root->val);
if(root->right)
s.push(root->right);
if(root->left)
s.push(root->left);
}
return ret;
}
};
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) {
vector<int> ret;
help(root, ret);
return ret;
}
void help(TreeNode* root, vector<int> &ret){
if (!root) return;
ret.push_back(root->val);
help(root->left, ret);
help(root->right, ret);
return;
}
};