LeetCode 144. Binary Tree Preorder Traversal

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

For example:
Given binary tree {1,#,2,3},
1
\
2
/
3
return [1,2,3].

/**
 * 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*> s;
        vector<int> v;
        if(root == NULL) {
            return v;
        }
        TreeNode *p = root;
        s.push(p);
        while(!s.empty()) {
            p = s.top();
            s.pop();
            v.push_back(p->val);
            if(p->right != NULL) {
                s.push(p->right);
            }
            if(p->left != NULL) {
                s.push(p->left);
            }
        } 
        return v;
    }
};

/**

* 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*> s;

vector<int> v;

if(root == NULL) {

return v;

}

TreeNode *p = root;

s.push(p);

while(!s.empty()) {

p = s.top();

s.pop();

v.push_back(p->val);

if(p->right != NULL) {

s.push(p->right);

}

if(p->left != NULL) {

s.push(p->left);

}

return v;

}

};

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