144.binary-tree-preorder-traversal.cpp

/*
 * @lc app=leetcode id=144 lang=cpp
 *
 * [144] Binary Tree Preorder Traversal
 *
 * https://leetcode.com/problems/binary-tree-preorder-traversal/description/
 *
 * algorithms
 * Medium (50.22%)
 * Total Accepted:    306K
 * Total Submissions: 609K
 * Testcase Example:  '[1,null,2,3]'
 *
 * Given a binary tree, return the preorder traversal of its nodes' values.
 *
 * Example:
 *
 *
 * Input: [1,null,2,3]
 * ⁠  1
 * ⁠   \
 * ⁠    2
 * ⁠   /
 * ⁠  3
 *
 * Output: [1,2,3]
 *
 *
 * Follow up: Recursive solution is trivial, could you do it iteratively?
 *
 */
/**
 * Definition for a binary tree node.
 */
#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> result;
        stack<TreeNode *> st;
        if (root)
            st.push(root);
        while (!st.empty())
        {
            TreeNode *node = st.top();
            st.pop();
            result.push_back(node->val);
            if (node->right)
                st.push(node->right);
            if (node->left)
                st.push(node->left);
        }
        return result;
    }
};