[Leetcode] Populating Next Right Pointers in Each Node

Given a binary tree

    struct TreeLinkNode {
      TreeLinkNode *left;
      TreeLinkNode *right;
      TreeLinkNode *next;
    }

Populate each next pointer to point to its next right node. If there is no next right node, the next pointer should be set toNULL.

Initially, all next pointers are set to NULL.

Note:

• You may only use constant extra space.
• You may assume that it is a perfect binary tree (ie, all leaves are at the same level, and every parent has two children).

For example,
Given the following perfect binary tree,

         1
       /  \
      2    3
     / \  / \
    4  5  6  7

After calling your function, the tree should look like:

         1 -> NULL
       /  \
      2 -> 3 -> NULL
     / \  / \
    4->5->6->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) {}
 * };
 */
class Solution {
public:
    /*algorithm level traversel based solution
    time O(n) space O(2^h) h is the tree height logn
    */
    void connect(TreeLinkNode *root) {
        if(!root)return;
        queue<TreeLinkNode*>Q;
        Q.push(root);
        while(!Q.empty()){
            int size = Q.size();
            for(int i = 0;i < size;i++){
                TreeLinkNode* t = Q.front();Q.pop();
                if(i+1 < size)t->next = Q.front();
                if(t->left)Q.push(t->left);
                if(t->right)Q.push(t->right);
            }
        }
    }
};

class Solution {
public:
    /*algorithm level order
    time O(n) space O(1) use prev establish level linked list
    so we can optmize the usage of queue, because previous level link list is just the queue 
    */
    void connect(TreeLinkNode *root) {
        if(!root)return;
        for(TreeLinkNode* first = root,* second = root->left;first && second;first = second,second = first->left){
                TreeLinkNode* f = first,*s = second;
                while(f){
                    if(f->left == s){
                        s->next = f->right;
                        f = f->next;
                    }else{
                        s->next = f->left;
                    }
                    s = s->next;
                }
        }
    }
};