114.flatten-binary-tree-to-linked-list.cpp

/*
 * @lc app=leetcode id=114 lang=cpp
 *
 * [114] Flatten Binary Tree to Linked List
 *
 * https://leetcode.com/problems/flatten-binary-tree-to-linked-list/description/
 *
 * algorithms
 * Medium (51.28%)
 * Likes:    4115
 * Dislikes: 401
 * Total Accepted:    433.5K
 * Total Submissions: 825.1K
 * Testcase Example:  '[1,2,5,3,4,null,6]'
 *
 * Given the root of a binary tree, flatten the tree into a "linked
 * list":
 * 
 * 
 * The "linked list" should use the same TreeNode class where the right child
 * pointer points to the next node in the list and the left child pointer is
 * always null.
 * The "linked list" should be in the same order as a pre-order traversal of
 * the binary tree.
 * 
 * 
 * 
 * Example 1:
 * 
 * 
 * Input: root = [1,2,5,3,4,null,6]
 * Output: [1,null,2,null,3,null,4,null,5,null,6]
 * 
 * 
 * Example 2:
 * 
 * 
 * Input: root = []
 * Output: []
 * 
 * 
 * Example 3:
 * 
 * 
 * Input: root = [0]
 * Output: [0]
 * 
 * 
 * 
 * Constraints:
 * 
 * 
 * The number of nodes in the tree is in the range [0, 2000].
 * -100 <= Node.val <= 100
 * 
 * 
 * 
 * Follow up: Can you flatten the tree in-place (with O(1) extra space)?
 */

// @lc code=start
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void flatten1(TreeNode* root) {
        if (!root) return;
        TreeNode dummy(0);
        TreeNode* pre = &dummy;
        stack<TreeNode*> st;
        st.push(root);
        while (!st.empty()) {
            TreeNode* node = st.top();
            st.pop();
            pre->left = nullptr;
            pre->right = node;
            pre = node;
            if (node->right) {
                st.push(node->right);
            }
            if (node->left) {
                st.push(node->left);
            }
        }
    }
    void flatten2(TreeNode* root) {
        TreeNode* node = root;
        while (node) {
            if (node->left) {
                TreeNode* r = node->left;
                while (r->right) {
                    r = r->right;
                }
                r->right = node->right;
                node->right = node->left;
                node->left = nullptr;
            }
            node = node->right;
        }
    }
    TreeNode* preNode = nullptr;
    void flatten(TreeNode* root) {
        if (root == nullptr)
            return ;
        flatten(root->right);
        flatten(root->left);
        root->right = preNode;
        root->left = nullptr;
        preNode = root;
    }
};
// @lc code=end