Apparently BFS is the most obvious one.. but it is not that simple - only constant extra space is provided.

Then the only strategy to take is recursion. I bogged down for quite while.. only after checked http://leetcode.com/2010/03/first-on-site-technical-interview.html, I got enlightment: pick a pen and a piece of paper, and simulate the procedure by drawing - this strategy is working so well on diagram-like problems! Just like "starting from enumeration and check pattern" strategy, a good solution starts from basics.

class Solution {public:    void connect(TreeLinkNode *p) {        if (!p) return;        if (p->left)    p->left->next = p->right;    // with the same parent        if (p->right)    p->right->next = (p->next) ? p->next->left : NULL;        connect(p->left);        connect(p->right);    }};