Populating Next Right Pointers in Each Node II

Follow up for problem "Populating Next Right Pointers in Each Node".

What if the given tree could be any binary tree? Would your previous solution still work?

Note:

• You may only use constant extra space.

For example,
Given the following binary tree,

         1
       /  \
      2    3
     / \    \
    4   5    7

After calling your function, the tree should look like:

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

使用一個隊列保存當前節點和當前層號，prev保存這個節點的前一個節點，如果節點屬於同一層，則prev的next指向隊列的頭元素。

層次遍歷

Have you been asked this question in an interview? 

/**
 * 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) {
        if(!root) return;
        queue< pair<TreeLinkNode*,int> > q;
        q.push(pair<TreeLinkNode*,int>(root,0));
        pair<TreeLinkNode* ,int> prev;
        while(!q.empty()){
            pair<TreeLinkNode*,int> temp=q.front();
            q.pop();
            if(prev.first!=NULL&& prev.second==temp.second)
                prev.first->next=temp.first;
            prev=make_pair(temp.first,temp.second);
            if(temp.first->left) q.push(pair<TreeLinkNode*,int>(temp.first->left,temp.second+1));
            if(temp.first->right) q.push(pair<TreeLinkNode*,int>(temp.first->right,temp.second+1));
        }
    }
};