populating next right pointers in each node ii

     1
   /  
  2    3
 /     
4   5    7

     1 -> NULL
   /  
  2 -> 3 -> NULL
 /     
4-> 5 -> 7 -> NULL

 * 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) {}
 * };
 */
void connect(TreeLinkNode *root) {
    while(root != NULL){
        TreeLinkNode* prev = NULL;
        TreeLinkNode* nxt = NULL;
        
        for(;root != NULL; root = root->next){
            if(nxt == NULL){
                //为叶子节点
                if(root->left == NULL && root->right == NULL) nxt = NULL;
                else{
                    nxt = (root->left != NULL ? root->left : root->right);
                }
            }
            
            if(root->left != NULL){
                if(prev != NULL){
                    prev->next = root->left;
                }
                prev = root->left;
            }
            
            if(root->right != NULL){
                if(prev != NULL){
                    prev->next = root->right;
                }
                prev = root->right;
            }
        }
        
        root = nxt;
    }
}