LeetCode144. 二叉樹的前序遍歷(Binary Tree Preorder Traversal)
阿新 • • 發佈:2018-11-14
題目描述
給定一個二叉樹,返回它的 前序 遍歷。
示例:
輸入: [1,null,2,3]
1
\
2
/
3
輸出: [1,2,3]
進階: 遞迴演算法很簡單,你可以通過迭代演算法完成嗎?
遞迴程式碼:
/**
* 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> result;
vector<int> preorderTraversal(TreeNode* root) {
if(root){
result.push_back(root->val);
preorderTraversal(root->left);
preorderTraversal(root->right);
}
return result;
}
};
非遞迴程式碼:
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
/**
* 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> res;
stack<TreeNode*> s;
//TreeNode *p=root;
while(root||!s.empty()){
if(root){
s.push(root);
res.push_back(root->val);
root=root->left;
}
else{
root=s.top();
s.pop();
root=root->right;
}
}
return res;
}
};