144. Binary Tree Preorder Traversal

• Difficulty: Medium

• Topics: Stack, Tree

• Similar Questions:

  • Binary Tree Inorder Traversal

  • Verify Preorder Sequence in Binary Search Tree

  • N-ary Tree Preorder Traversal

Problem:

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?

Solutions:

/**
 * 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> ret;

        TreeNode* cur = root;
        while (cur) {
            TreeNode* pre = cur->left;

            if (pre == nullptr) {
                ret.push_back(cur->val); // visit
                cur = cur->right;
                continue;
            }

            while (pre->right != nullptr && pre->right != cur) {
                pre = pre->right;
            }

            if (pre->right == nullptr) {
                pre->right = cur;
                ret.push_back(cur->val); // visit
                cur = cur->left;
            } else {
                pre->right = nullptr;
                cur = cur->right;
            }
        }

        return ret;
    }
};