leetcode 刷题之路 81 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 to NULL.

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

Given a binary tree

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

二叉树的结构体里增加了next指针，编写程序，将二叉树里每个节点的next指针指向它右边的节点。

思路，对二叉树进行层序遍历，使用previous指针记录上一个节点，遍历到当前节点时将previous的next指向当前节点，并将previous改为当前节点进行下一次循环；使用num1，num2来记录每层的节点个数，来区分每层的界限，更具体的逻辑可以参考程序。

代码如下：

/**
 * 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==NULL)
            return;
        queue<TreeLinkNode*> qt;
        TreeLinkNode *previous=NULL;
        int num1=1,num2=0;
        qt.push(root);
        while(!qt.empty())
        {
            while(num1>0)
            {
                root=qt.front();
                qt.pop();
                num1--;
                if(root->left!=NULL)
                {
                    qt.push(root->left);
                    num2++;
                }
                if(root->right!=NULL)
                {
                    qt.push(root->right);
                    num2++;
                }
                if(previous!=NULL)
                    previous->next=root;
                previous=root;
            }
            previous->next=NULL;
            previous=NULL;
            num1=num2;
            num2=0;
        }
    }
};

leetcode 刷题之路 81 Populating Next Right Pointers in Each Node