leetcode

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].

思路：前序遍历，“根，左子树，右子树”

#include <iostream>
#include <vector>
#include <stack>
using namespace std;

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;
        if (!root) {
            return res;
        }

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

int main() {
    return 0;    
}