Populating Next Right Pointers in Each Node

solution.cpp

/**
 * Definition for binary tree with next pointer.
 * struct TreeLinkNode {
 *  int val;
 *  TreeLinkNode *left, *right, *next;
 *  TreeLinkNode(int x) : val(x), left(NULL), right(NULL), next(NULL) {}
 * };
 */
class Solution {
public:
    void connect(TreeLinkNode *root) {
        auto p = root;
        
        // Loop invariant: p is always the 'left most' node at the current level
        while (p != NULL) {
            // q is used to walk through from left to right at the current level
            auto q = p;
            while (q != NULL) {
                if (q -> left != NULL)
                    q -> left -> next = q -> right;
                if (q -> next != NULL && q -> right != NULL)
                    q -> right -> next = q -> next -> left;
                q = q -> next;
            }
            p = p -> left;
        }
    }
};