116. Populating Next Right Pointers in Each Node

Recursive Solution

• self explanatory

Code

class Solution {
private:
    void connectDFS(Node* root)
    {
        if (root == NULL || root->left == NULL) // check if root or left is NULL
            return;

        root->left->next = root->right; // next pointer of root's left

        if (root->next != NULL) // if root's next exist
            root->right->next = root->next->left; // next pointer of root's right

        connectDFS(root->left);
        connectDFS(root->right);
    }

public:
    Node* connect(Node* root)
    {
        connectDFS(root);
        return root;
    }
};

Iterative Solution

• Self explanatory

Code

class Solution {
public:
    Node* connect(Node* root)
    {
        if (root == NULL) return NULL;

        queue<Node*> q;
        q.push(root);

        while (!q.empty()) {
            int sz = q.size();
            for (int i = 0; i < sz; i++) {
                Node* temp = q.front(); q.pop();
                if (temp->left) q.push(temp->left);
                if (temp->right) q.push(temp->right);

                // if it is last node, it's next is NULL else q.front()(next left node)
                temp->next = (i == sz - 1 ? NULL : q.front());
            }
        }
        return root;
    }
};

O(N) Time and O(N) space

• Using level order traversal technique and NULL.
• if current node is null and q is not empty, then push NULL into q.
• else set current node’s next to q’s front.
• push left and right in the queue , if they are not NULL.

Code

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

        queue<Node *> q;
        q.push(root);
        q.push(NULL);

        while(!q.empty()){
            Node *curr=q.front();
            q.pop();

            if(curr==NULL){
                if(!q.empty()) q.push(NULL);
            }else{
                curr->next = q.front();
                if(curr->left!=NULL) q.push(curr->left);
                if(curr->right!=NULL) q.push(curr->right);
            }
        }
        return root;
    }
};

O(N) Time and O(1) space

• level order traversal with root and its child,just dry run once you get the idea.

Code

class Solution{
public:
    Node *connect(Node *root){
        if (root == NULL) return root;

        Node *pre = root;
        Node *cur = NULL;

        while (pre->left){
            cur = pre;
            while (cur){
                cur->left->next = cur->right;
                if (cur->next)
                    cur->right->next = cur->next->left;
                cur = cur->next;
            }
            pre = pre->left;
        }
        return root;
    }
};