144. Binary Tree Preorder Traversal

问题描述

Given a binary tree, return the preorder traversal of its nodes’ values.

For example:

Given binary tree [1,null,2,3],

1

\

2

/

3

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

解题思路

该问题比较明确，给出一个二叉树，要求我们用前序遍历遍历树的节点。并且要求用非递归的方式。我们只要借用栈结构，用前序的方式遍历二元树并将节点的值存入向量中即可。

代码展示

/**
* 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*> stack_pre;
if(root==NULL)
return res;
stack_pre.push(root);
while(!stack_pre.empty()){
TreeNode* top_node=stack_pre.top();
res.push_back(top_node->val);
stack_pre.pop();
if(top_node->right!=NULL)
stack_pre.push(top_node->right);
if(top_node->left!=NULL)
stack_pre.push(top_node->left);
}
return res;
}
};

结果展示