116.Populating Next Right Pointers in Each Node

class Solution {
public:
    Node* connect(Node* root) {
        if (!root) return NULL;
        if (root->left) root->left->next = root->right;
        if (root->right) root->right->next = root->next? root->next->left : NULL;
        connect(root->left);
        connect(root->right);
        return root;
    }
};

// Non-recursion, more than constant space
class Solution {
public:
    Node* connect(Node* root) {
        if (!root) return NULL;
        queue<Node*> q;
        q.push(root);
        while (!q.empty()) {
            int size = q.size();
            for (int i = 0; i < size; ++i) {
                Node *t = q.front(); q.pop();
                if (i < size - 1) {
                    t->next = q.front();
                }
                if (t->left) q.push(t->left);
                if (t->right) q.push(t->right);
            }
        }
        return root;
    }
};