114. Flatten Binary Tree to Linked List

/**
 * 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:
    void flatten(TreeNode* root) {
        helper(root);
    }

    TreeNode *helper(TreeNode *root) { // 返回flatten之后的最后一个结点
        if (!root) return root;
        auto l = helper(root->left); // 要保证l和r至少有一个不为空
        auto r = helper(root->right);
        if (l) { // 如果l不为空，root->right要接到l右边
            l->right = root->right;
            root->right = root->left;
            root->left = nullptr;
        }
        return r ? r : l ? l : root; // 如果r也不为空，返回r，如果l不为空，返回l，否则返回root
    }
};

/**
 * 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:
    void flatten(TreeNode* root) {
        flatten_helper(root);
    }

private:
    TreeNode *flatten_helper(TreeNode *root) {
        if (!root || !root->left && !root->right)
            return root;
        auto left_last(flatten_helper(root->left));
        if (left_last) {
            left_last->right = root->right;
            root->right = root->left;
            root->left = nullptr;
        }
        return left_last ? flatten_helper(left_last) : flatten_helper(root->right);
    }
};