116. Populating Next Right Pointers in Each Node

• recursion

/*
// Definition for a Node.
class Node {
public:
    int val;
    Node* left;
    Node* right;
    Node* next;

    Node() : val(0), left(NULL), right(NULL), next(NULL) {}

    Node(int _val) : val(_val), left(NULL), right(NULL), next(NULL) {}

    Node(int _val, Node* _left, Node* _right, Node* _next)
        : val(_val), left(_left), right(_right), next(_next) {}
};
*/

class Solution {
public:
    Node* connect(Node* root)
    {
        if (!root) return nullptr;

        if (root->left)
        {
            root->left->next = root->right;
        }

        if (root->right)
        {
            root->right->next = root->next ? root->next->left : nullptr;
        }

        connect(root->left);
        connect(root->right);
        return root;
    }
};

• T: $O(n)$

• S: $O(n)$

• Iteration

/*
// Definition for a Node.
class Node {
public:
    int val;
    Node* left;
    Node* right;
    Node* next;

    Node() : val(0), left(NULL), right(NULL), next(NULL) {}

    Node(int _val) : val(_val), left(NULL), right(NULL), next(NULL) {}

    Node(int _val, Node* _left, Node* _right, Node* _next)
        : val(_val), left(_left), right(_right), next(_next) {}
};
*/

class Solution {
public:
    Node* connect(Node* root)
    {
        if (!root) return nullptr;

        queue<Node*> q{{root}};

        while (!q.empty())
        {
            int qSize = q.size();
            for (int i = 0; i < qSize; i++)
            {
                Node* node = q.front(); q.pop();
                if (i < qSize - 1)
                {
                    // 如果不是最末端的 node，node->next 指到下一個節點
                    node->next = q.front();
                }

                if (node->left) q.push(node->left);
                if (node->right) q.push(node->right);

            }
        }
        return root;
    }
};

• T: $O(n)$
• S: $O(n)$